Fix RDC error when chains of molecules are mismatches

[Justin Lorieau]

When chains of input molecules are mismatched, the following error occurs:

Traceback (most recent call last):
...
   File "~/Code/Python/BiophysicsAndBiochemistry/mollib/mollib/pa/svd.py", line 147, in calc_pa_SVD
    U, w, V = linalg.svd(A, full_matrices=False)
  File "~/Code/Python/VirtualEnvironments/mollib36/lib/python3.6/site-packages/scipy/linalg/decomp_svd.py", line 98, in svd
    raise ValueError('expected matrix')
ValueError: expected matrix