NF: add mmap keyword to ArrayProxy class

Give array proxy class ability to tune memory mapping via `mmap`
keyword-only argument.
NF: add mmap keyword for Analyze image types

Allow specifying mmap flags when loading Analyze-type images.
NF: top-level load passes on keyword arguments

Check this works for mmap keyword argument to top-level load.
RF: refactor arrayproxy tests for re-use

I want to use the arrayproxy tests for other ArrayProxy objects,
particularly the PARRECArrayProxy. Refactor to make this easier.
RF: refactor image mmap test for re-use

Refactor image arrayproxy test mixin to make it easier to deal with the
case where we can't easily make a new image from scratch, and we might
want to use some example image instead.
NF: add mmap keyword to PARREC with tests

Use refactored tests to test PARREC mmap interface.