Add APBS output file reader

This adds a new extension for reading output files (in the OpenDX
format) from APBS.

Change-Id: I91b15a5d1a8145679c69a71307c7ce53b08c26f4
Minor cleanups

Change-Id: I7026064cdda9cc59ea7d9c3eaf963f522a6ca37c
Merge remote-tracking branch 'origin/master' into apbs-output

Conflicts:
	avogadro/qtplugins/CMakeLists.txt

Change-Id: I14b6f2a30effc4e603f2b508941e2395c97e255d
Merge topic 'apbs-output' into master

37de7964 Merge remote-tracking branch 'origin/master' into apbs-output
17d629cd Minor cleanups
c39cdf8b Add APBS output file reader