RF: move mincfile extraction logic into __init__

Let the init do all the work of getting the variables out of the
mincfile, so that the other methods can work for MINC1 and MINC1
RF: continue MINC1 refactoring for adding MINC2

Move more logic into __init__ of Minc1File

Make compressed tests an addition to a base test class; MINC2 cannot use
compressed files.

Some renaming of logic for creating loaded file from file objects.
NF: First pass at MINC2 loading with tests

Add MINC2 loading - very basic, with no save.
Inherit MINC1 tests
Add example file and check loaded parameters against mincstats