NF: add assert_data_similar function

Thinking about testing real example images against recorded information
about these images, maybe from previous loads, or loads by other
software.

This helper function tests against recorded (previous) loaded data if
present, or recorded summary statistics otherwise.
RF: refactor image API classes

Refactor the image API testing classes to:

* make it clearer how to add one;
* allow testing with example images.
RF+TST: use new API to test MINC image load

Refactor MINC tests to use example image format from the image API
validation code.

Add these these examples to the image API validation tests.
RF+TST: add image API tests for PARREC

Use image validator API to test API of loaded PARREC images.
Merge pull request #262 from matthew-brett/refactor-api-validators

MRG: refactor API validation code for clarity

Refactor API validation code to make it clearer how to add another image to the
validation. Refactor PARREC and MINC to give examples of testing image APIs
for images that are load-only.