Cleanup

[op3]

A general cleanup of the code:

• Move headers to alpaca subdirectory
• Use alpaca namespace (C++ and cmake)
• Parity and EMCharacter as enum class for strong typing.
• Named constructors for Dipole, Quadrupole, E1, M1, E2, M2 transitions and States
• Typing for euler, spherical, cartesian coordinates.
• As a consequence, the sampler methods now return samples of coordinates (one vector) instead of samples of coordinates (several vectors), which also makes more sense.
• Only three libraries with a better defined scope: coeff, angcorr, sampler
• pybind11-based python bindings.
• The old cytpes-based bindings were removed.
• Easy installation using pip.