Fix bug in get_coords
Fix bug in get_coords

get_coords() currently returns a list of coordinate tuples where the third position of each tuple is the entire list of z positions rather than the corresponding z position. For example:

In [70]: pdb_2dog.get_coords()[0]
Out[70]: 
(-16.847000000000001,
 2.899,
 array([ 8.705,  7.462,  7.05 , ..., -7.325, -8.086, -7.866]))
rcsb/mmtf-python