add c++ angle axis CoordsAdaptor
add untested aa to rotation matrix function

also add a hacky matrix implementation
forgot to add the new angle axis otp tests
import some angle axis changes I made in another branch

remove unused folder pele/potentials/fortran/rigidbodies

rmdrvt.f90 duplicates a function that is in angleaxis/_aadist.f90

add file _aa_utils.py with some tests
add a test and fix some bugs in aa_to_rot_mat
add RigidFragment with to_atomistic()

and a test
add c++ RBTopology and to_atomistic

along with tests
add c++ rotation matrix derivatives

and tests
add c++ site transform_grad

with tests
add c++ RBTopology::transform_gradient
add c++ RBPotentialWrapper

and tests
add c++ angle axis RBPotentialWrapper

the c++ code is wrapped in cython.

the cython compiler couldn't find _pele.pyx, so I modified setup.py and
cythonize.py so pele/potentials/ could be passed to cython as an include
directory
add documentation to new c++ angle axis stuff

also do a bit of clean up
add small theta version of c++ rot_mat_derivatives()
improve EXPECT_NEAR_RELATIVE
move some function implementations to aatopology.cpp

do a bit of cleanup
forgot to include aatopology.cpp in setup.py
return by reference in HackyMatrix
add a bit of documentation
Merge branch 'master' of https://github.com/pele-python/pele into new_hswca