Expose the default parameter sets for each imexam() function and allow users to set their own defaults

[sosey]

Users can already view and set the parameters for each function (which are stored as a dictionary) but it would probably be useful to install a parameters file somewhere which they can edit to override the defaults.

[pllim]

You already use astropy_helpers, so I think the natural thing to do here is to use the astropy.config functionality. Just a thought.

[sosey]

@pllim I'm not sure that it's the natural interface to the default parameter sets I'm using for the plotting/information routines, which are not set up as simple key-value pairs, but rather dictionaries of associated settings and descriptions for each function which is implemented. I'll take a closer look through the config code astropy uses though.

[pllim]

You can also look at how Ginga implements plugin configuration files, which are plugin-specific.