Provide cpp module

[henryiii]

There are some pythonisations (mostly related to arrays and indexing) that duplicate the C++ API. We could add a boost_histogram.cpp module, and this would provide the exact Boost.Histogram API where possible.

Things to move or provide there:

.at(i) # In Python: [] and bh.underflow/overflow
.axis(i) # Tuple `.axes` in Python

This might be supportable alongside boost_histogram, or it might inject the methods directly to the existing classes. I'm not sure I'm very excited about creating a new collection of classes -- but if it can be done carefully, it might work (Basically we need Ruby's extend...).

[henryiii]

@HDembinski, would you be okay with me hiding .at -> ._at and .axis -> ._axis in 0.5.1 so that no one starts depending on these in the main package?

[henryiii]

Corrected plan: In 0.5.2, the .at and .axis would still exist, but would raise a DepricationWarning and then call ._at and ._axis. In 0.6.0, these would be added or made available in compat, without a warning, and would disappear from the non-compat version.

[HDembinski]

That is fine!