Tweaks to methylformate example input file.

It now only runs PDep calcs on networks with up to 16 atoms,
and it saves the restart file every hour.
Tweak requirements.txt file for python dependencies.

The default version of pyparsing, which is installed
as a sub-dependency, only works on Python 3+. We now
explicitly install an older version, so that the other
dependencies work on Python 2.x.