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