rewritten hs-wca potential

the hs-wca potential has been rewritten such that no square roots need to be computed, these changes should improve performance. Note that the functional form of the potential is marginally different now.
removed extra level of indirection from operator[], now returning a copy. For doubles copy is cheap
add c++ angle axis CoordsAdaptor
remove using:: and added coordinates file for test
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
bug fix to hessian and fixed tests
add c++ site transform_grad

with tests
Merge branch 'master' of https://github.com/pele-python/pele into new_hswca
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()
move some function implementations to aatopology.cpp

do a bit of cleanup