Merge pull request #65 from kandersolar/installation-doc

Installation: recommend `[optional]` instead of `[all]`

Author: AdamRJensen

docs/source/installation.md

@@ -6,6 +6,6 @@ The package can be installed from [PyPI](https://pypi.org/project/solposx/) with
 
 Some functions require additional optional dependencies, e.g., the Python packages [skyfield](https://pypi.org/project/skyfield/) and [sg2](https://pypi.org/project/sg2). For convenience, the optional dependencies can be installed with the following command:
 
-    pip install solposx[all]
+    pip install solposx[optional]
 
 The source code is openly developed on [GitHub](https://github.com/AssessingSolar/solposx).