Move all instances out of gistCore into individual files

[rjyounes]

E.g., move all units into a gistUnits file, mime types into gistMimeTypes, etc. Users will have to import the files they need.

[rjyounes]

Relates tangentially to #463.

[uscholdm]

IRelates directly (by subsumption?) to #305 . We might want to introduce a gistx namespace.

[rjyounes]

Will not fix. There are classes referring to the instances, so they should not be moved out into a separate file - would create a circular import.