Need to modify default input parameter based on deal.II changes in v9.2.0

[stvdwtt]

Created in response to this forum post: https://groups.google.com/g/prisms-pf-users/c/IQQ_5n3dGbE

In v9.2.0 deal.II now mandates that input files end with the ".prm" extension. This is obviously in conflict with our convention of using the ".in" extension. We could create a custom subclass of their input parser, but the best choice is probably just to change our file extensions.

The default input file name is given here: https://github.com/prisms-center/phaseField/blob/master/include/ParseCommandLineOpts.h

If we change this, we'd probably also want to have an exception handler for when a "parameters.in" file exists but a "parameters.prm" file doesn't. That way the user gets a more informative exception message than what Pushkar received.

@david-montiel-t, do you think you can take care of this?