Merge pull request #1438 from giacomofiorin/colvars-update

to the system's potential energy as part of "thermodynamic

output"_thermo_style.html.

The {fix_modify configfile <config file>} option allows to add settings

from an additional config file to the colvars module. This option can

only be used, after the system has been initialized with a "run"_run.html

command.

The {fix_modify config <quoted string>} option allows to add settings

from inline strings. Those have to fit on a single line when enclosed

in a pair of double quotes ("), or can span multiple lines when bracketed

by a pair of triple double quotes (""", like python embedded documentation).

This fix computes a global scalar which can be accessed by various

"output commands"_Howto_output.html.  The scalar is the cumulative

energy change due to this fix.  The scalar value calculated by this