Modifying plugin arguments

[kjdon]

It would be nice to add in to PrintInfo (or other place?) some methods to modify plugin arguments.

Useful things:

Change the default value for a particular argument. This way plugins can override the defaults for super class arguments. This should get rid of the need to have process_exp arg in each plugin.

Make an argument disabled, with a reason, which GLI could then display. For example, ImageCOnverter could disable all its arguments if ImageMagick is no where to be found. Similarly for GISExtractor, if the gis stuff is not installed.