Make gmx-gangle more flexible with multiple angles

Add support for providing only a single selection and/or a single vector
while the second group of selections is larger.  In such cases, expand
the smaller selection to match the size of the larger selection.

This makes it easier to compute multiple angles against the same vector.
For example, computing the relative angle of N molecules against a
single reference molecule.

Change-Id: I464acc8655c1bae09a88dff7d7301d20712d270e