WIP: Replace swig

[ashermancinelli]

@intendo @sobolevnrm Please leave review comments on the API I've exposed here. Let me know if this is how you'd like to interact with the C code from the python side - since we're already replacing the SWIG interface, this would be a good time to air your wishlist. Currently working example:

$ # make sure you have both swig and pybind11 installed for now while we gradually replace swig
$ cmake .. -DENABLE_PYTHON=ON -DENABLE_TOOLS=ON
$ make -j
$ cd lib
$ python3
Python 3.8.5 (default, Jul 21 2020, 10:48:26)
[Clang 11.0.3 (clang-1103.0.32.62)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from apbs import NOsh
>>> nosh = NOsh()
>>> nosh.parseInputFromString(...)

From here, we can easily expose any of the C code as a clean and intuitive OO interface, but if either of you would like something different, please comment here.

• @intendo's comments
• @sobolevnrm's comments
• API review
• highest-priority kernels exposed through python
  • (please comment which kernels should go here)

[intendo]

@ashermancinelli, how do you install pybind11 on Windows, Linux, and MacOSX? I can't get it working in Github Actions (build.xml) and Appveyor (.appveyor). Did you build pybind11 from source?

[ashermancinelli]

@intendo I believe it's a header only lib. Best option is probably too add it as a git submodule.