Minor refactoring of mopac.py

Changing some if/else blocks to reduce nesting.
If we check the error condition (which will immediately return False) first,
then the rest doesn't need to be nested in an else block, and it's easier to
read from start to end and understand the flow control.
Minor refactoring of gaussian.py

Changing some if/else blocks to reduce nesting.
This is very much like the commit I just made in mopac.py
I wonder if some of this code duplication can be reduced...
Added @gmagoon's comments to the Gaussian pm3 keywords list.

In doing so I noticed that one of the sets of keywords was wrong 
(it was a duplicate of the line above it).