Updated dihedral.py and parsers.py to allow user-defined dihedrals.

I've tested this out on my projects and it seems to work well.   Perhaps you other developer-types could take a look to confirm.     Only minor changes to metrics/dihedral.py and metrics/parsers.py were required. --Vince

<pre>
$ Cluster.py dihedral -h

...optional arguments:
  -h, --help            show this help message and exit
  -a DIHEDRAL_ANGLES    which dihedrals. Choose from phi, psi, chi (to choose
                        multiple, seperate them with a slash), or user
                        Default: phi/psi
  -f DIHEDRAL_USERFILENAME
                        filename for dihedral indices, N lines of 4 space-
                        separated indices (otherwise ignored) Default:
                        DihedralIndices.dat
  -p DIHEDRAL_P         p used for metric=minkowski (otherwise ignored)
                        Default: 2
</pre>