Fixed CMAP generation for alanine dipeptide special residue

Fix CMAP to work for residues not detected as proteins, and
make sure we correctly process single-residue entries that
have residue-internal CMAP definitions. In particular, this
is important for the default alanine dipeptide special
residues (ALAD) in Charmm27.

Fixes #824.

Change-Id: I5538b26175f12dcac983491323b5b6ee303c8434