SCIFIO: limit default constructor services

When no context is provided to a SCIFIO constructor, it now defaults to
using just SCIFIOServices and SciJavaServices.
ImgOpener: standardize SubRegion check

Added a helper method to check if it's safe to constrain specified axis
given a particular SubRegion.

This should standardize the constraint process and fix bugs where
constraints were applied out of order.

Closes #58