__add__ and __iadd__

[robertodr]

Another possible fix for #101.

Context

In principle __add__ and __iadd__ should do the exact same thing, so in C++ one would overload operator+ and then overload operator+= using operator+. Things should be done similarly in practice. Up to now the two functions where using different implementations.

Possible solutions

1: Fix the bug in the implementation of __iadd__

Even though we can make the two different implementations do the exact same thing, I'd still argue that the strategy is un-intuitive and possibly bug-prone.

2: Implement __add__ in the bindings and attach __iadd__ dynamically to the class in Python

The strategy is:

• Remove __iadd__ from trees.h.
• Add a trees.py file in src/vampyr containing:

from ._vampyr.vampyr3d import FunctionTree

def _impl__iadd__(...):
    ....

FunctionTree.__iadd__ = _impl__iadd__

• Modify __init__.py like so:

from .trees import FunctionTree

I tried this, but it actually adds more code complexity Python-side! Part of the reason is that Python does not have a notion of templates, so attaching a method to FunctionTree has to be done once per value of D (3 times as things stand now) And actually instead of a single trees.py this needs to be done in 3 separate Python files plus the re-export in __init__.py loses all notions of namespacing by dimension (vampyr1d, vampyr2d, vampyr3d) I think this is hard to get right.

3: Implement __add__ as a free function instead of a lambda and re-use it for __iadd__

This is what I ended up doing in this PR.

[robertodr]

The diff is a bit too large between this branch and master, even after rebasing 🤷🏻

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +8.49 🎉

Comparison is base (bbe2319) 82.41% compared to head (8d44a79) 90.90%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   82.41%   90.90%   +8.49%     
==========================================
  Files          22        3      -19     
  Lines         910      187     -723     
==========================================
- Hits          750      170     -580     
+ Misses        160       17     -143     

see 19 files with indirect coverage changes

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[bjorgve]

LGTM

Thanks, Roberto!

[robertodr]

Note that all other overloaded operators should be double-checked (and in case re-implemented)