Add instructions to install on apple silicon (#335)

Author: dnerini

doc/source/user_guide/install_pysteps.rst

@@ -45,45 +45,78 @@ Other optional dependencies include:
 But, if you want to contribute to pysteps or edit the package, you need to install
 pysteps in development mode: :ref:`Contributing to pysteps <contributor_guidelines>`.
 
-Anaconda install (recommended)
-------------------------------
-
-Conda is an open-source package management system and environment management
-system that runs on Windows, macOS, and Linux.
-Conda quickly installs, runs, and updates packages and their dependencies.
+Install with conda/mamba (recommended)
+--------------------------------------
+
+`Conda <https://docs.conda.io/>`_ is an open-source package management system and environment
+management system that runs on Windows, macOS, and Linux.
+`Mamba <https://mamba.readthedocs.io/>`_ is a drop-in replacement for conda offering
+better performances and more reliable environment
+solutions. Mamba quickly installs, runs, and updates packages and their dependencies.
 It also allows you to easily create, save, load, or switch between different
 environments on your local computer.
 
-Since version 1.0, pysteps is available in conda-forge, a community-driven
-package repository for anaconda.
-
-There are two installation alternatives using anaconda: install pysteps in a
-pre-existing environment or install it new environment.
-
-New environment
-~~~~~~~~~~~~~~~
+Since version 1.0, pysteps is available on `conda-forge <https://conda-forge.org/>`_,
+a community-driven package repository for conda packages.
 
-In a terminal, to create a new conda environment and install pysteps, run::
+To install pysteps with mamba in a new environment, run in a terminal::
 
-    $ conda create -n pysteps
-    $ source activate pysteps
+    mamba create -n pysteps python=3.10
+    mamba activate pysteps
 
-This will create and activate the new python environment. The next step is to
-add the conda-forge channel where the pysteps package is located::
+This will create and activate the new python environment called 'pysteps' using python 3.10.
+The next step is to add the conda-forge channel where the pysteps package is located::
 
-    $ conda config --env --prepend channels conda-forge
+    conda config --env --prepend channels conda-forge
 
 Let's set this channel as the priority one::
 
-    $ conda config --env --set channel_priority strict
+    conda config --env --set channel_priority strict
 
 The latter step is not strictly necessary but is recommended since
-the conda-forge and the default Anaconda channels are not 100% compatible.
+the conda-forge and the default conda channels are not 100% compatible.
 
 Finally, to install pysteps and all its dependencies run::
 
-    $ conda install pysteps
+    mamba install pysteps
+
+Install pysteps on Apple Silicon Macs
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+On conda-forge, pysteps is currently compiled for Mac computers with Intel processors (osx-64).
+However, thanks to `Rosetta 2 <https://support.apple.com/en-us/HT211861>`_ it is
+possible to install the same package on a Mac computers with an Apple Silicon processor
+(arm-64).
+
+First, make sure that Rosetta 2 is installed::
+
+    softwareupdate --install-rosetta
+
+Use mamba to create a new environment called 'pysteps' for intel packages with python 3.10::
+
+    CONDA_SUBDIR=osx-64 mamba create -n pysteps python=3.10
+    mamba activate pysteps
+
+Make sure that conda/mamba commands in this environment use intel packages::
 
+    conda config --env --set subdir osx-64
+
+Verify that the correct platform is being used::
+
+    python -c "import platform;print(platform.machine())"  # Should print "x86_64"
+
+Finally, run the same pysteps install instructions as given above::
+
+    conda config --env --prepend channels conda-forge
+    conda config --env --set channel_priority strict
+    mamba install pysteps
+
+We can now verify that pysteps loads correctly::
+
+    python -c "import pysteps"
+
+Note that the first time that pysteps is imported will typically take longer, as Rosetta 2
+needs to translate the binary code for the Apple Silicon processor.
 
 Install from source
 -------------------
@@ -93,8 +126,6 @@ to adhere to the `PEP517 standards <https://www.python.org/dev/peps/pep-0517/>`_
 Using ``pip`` instead of ``setup.py`` guarantees that all the package dependencies
 are properly handled during the installation process.
 
-.. _install_osx_users:
-
 OSX users: gcc compiler
 ~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -134,7 +165,6 @@ the homebrew installation. For example::
     export CC=/usr/local/Cellar/gcc/8.3.0/bin/gcc-8
     export CXX=/usr/local/Cellar/gcc/8.3.0/bin/g++-8
 
-
 Then, you can continue with the normal installation procedure described next.
 
 Installation using pip