Make a "help" flag for mpetrun and mpetplot

[Raymond Smith]

This could tell users (a) how to run/use them and (b) where to download default config files in case the package was installed rather than git-cloned.

[Raymond Smith]

mpettest:

1. Could have its current flag done the same way
2. Could have a flag to generate all the plots (useful if tests failed).

mpetplot:

1. Could be converted so that its arguments are parsed in a much more sane fashion using the library.

[Dan Cogswell]

Addressed in 46e4f52 with argparse.