refactor: drop cpp (#402)

* refactor!: drop CPP module

* refactor: removing BaseHistogram

* refactor: Simplify axes

Author: henryiii

docs/api/boost_histogram.cpp.axis.rst

@@ -8,7 +8,6 @@ boost\_histogram
 
 
 .. include:: boost_histogram.axis.rst
-.. include:: boost_histogram.cpp.rst
 
 
 boost\_histogram.accumulators

docs/api/boost_histogram.cpp.rst

@@ -4,9 +4,9 @@ Comparison with Boost.Histogram
 ===============================
 
 ``boost-histogram`` was based on the C++ library Boost.Histogram. In most ways,
-it mimics the interface of this library; if you learn to use one, you probably can use
-the other. A special high-compatibility mode is available with ``boost_histogram.cpp``,
-as well, which reduces the changes to a bare minimum. There are a few differences, however:
+it mimics the spirit of this library; if you learn to use one, you probably can use
+the other. There are a few differences, however, mostly around adhering to Python
+conventions:
 
 Removals
 ^^^^^^^^
@@ -21,14 +21,12 @@ The call operator
    fills are not encouraged in Python due to poor performance. The ``.fill`` method from
    Boost.Histogram 1.72 is bound, however - this provides fast fills without the drawbacks.
    If you want to fill with a single item, Python's ``.fill`` does support single item fills.
-   ``cpp`` mode does have a call operator.
 
 Histogram make functions
    These functions, such as ``make_histogram`` and ``make_weighted_histogram``, are provided
    in Boost.Histogram to make the template syntax easier in C++14. In C++17, they are replaced
    by directly using the ``histogram`` constructor; the Python bindings are not limited by old
    templating syntax, and choose to only provide the newer spelling.
-   ``cpp`` mode does have the make functions.
 
 Custom components
    Many components in Boost.Histogram are configurable or replaceable at compile time; since
@@ -37,20 +35,36 @@ Custom components
 Changes
 ^^^^^^^
 
+Naming
+   The bindings follow modern Python conventions, with CamelCase for classes,
+   etc. The Boost.Histogram library follows Boost conventions.
+
 Serialization
-   The Python bindings use a pickle based binary serialization, so cannot read files saved in C++ using Boost.Serialize.
+   The Python bindings use a pickle-based binary serialization, so cannot read
+   files saved in C++ using Boost.Serialize.
 
 Properties
-   Many methods in C++ are properties in Python. In ``cpp`` mode, most of these remain methods, except for ``.metadata``,
-   which needs to be a property to allow it to be set.
+   Many methods in C++ are properties in Python. ``.axis(i)`` is replaced with ``.axes[i]``.
+
+Indexing
+  The Python bindings use standard Python indexing for selection and setting.
+  You can recover the functionality of ``.at(i)`` at endpoints with
+  ``bh.tag.at(i)``.
 
+Renames
+  The ``.rank()`` method is replaced by the ``.ndim`` property to match the common NumPy spelling.
 
 Additions
 ^^^^^^^^^
 
 Unified Histogram Indexing
-   The Python bindings support UHI, a proposal to unify and simplify histogram indexing in Python. Currently,
-   this is disabled in ``cpp`` mode.
+   The Python bindings support UHI, a proposal to unify and simplify histogram
+   indexing in Python.
+
+Custom transforms
+   Custom transforms are possible using Numba or a C pointer. In
+   Boost.Histogram, you can use templating to make arbitrary transforms, so a
+   run time transform is not as necessary (but may be added).
 
-Numpy compatibility
-   The Python bindings do several things to simplify Numpy compatibility.
+NumPy compatibility
+   The Python bindings do several things to simplify NumPy compatibility.