Tests for reslicing

Increasing coverage and adding tests for reslicing as promised!


I have been thinking for long time that the function name (resample) and file name (aniso2iso.py) were not good names. Not descriptive enough. What this function is doing is actually reslicing and this PR is correcting that. But at the same time some people maybe prefer aniso2iso. So, in that way I keep both. I am happy for suggestions.