numgrid v2.0.0

The API changed for easier maintenance and simpler use:

• No initialization or deallocation necessary.
• One-step instead of two steps (since the radial grid generation time is
  negligible compared to space partitioning, it did not make sense anymore to
  separate these steps and introduce a state).
• alpha_min is given as dictionary which saves an argument and simplifies
  explaining the API.
• The library now provides Rust and Python bindings. It used to provide C and
  Fortran bindings. The C/Fortran code lives on on the `cpp-version\ branch. I might bring the
  C interfaces back into the Rust code if there is sufficient interest/need.
• Note that the API will probably change again as soon as support for more
  quadratures is added (see #43).