RF: refactor general proxy API into own class

Put general API testing into its own class ready to test other array
proxy classes (such as MINC)
NF: add read-only shape to Minc array proxy

Writing API validators revealed shape was not read-only.
RF: rename test for minc2

Test still had minc1 naming
TST: add minc1 / minc2 proxy tests

Use generalized test framework to add minc1, minc2 array proxy tests
NF: add is_proxy attribute
TST+RF: add ecat tests with read-only attrs
RF+TST: make Analyze slope, inter, offset public

We may want to look at the slope, intercept, offset directly.  I can
imagine situations where you'd want to change slope, inter, to get free
rescaling of the images.  I don't know whether that's a good idea - to
make it part of the public API.