redesign similarty measure object to allow different normalization strategies
update affine registration example
fixed python 3 issue
allow input transforms as arrays for resampling
some renaming for clarity
add test
avoid modifying input arg in explore method of HistogramRegistration
add gradient and hessian computation methods to HistogramRegistration object
pep8 editing
add docstrings
rename flag argument normalize==True as renormalize==False for clarity
add test for gradient and Hessian computation of registration similarity function
modifs suggested by Matthew
fix clumsy test
Merge pull request #254 from alexis-roche/refactor-registration-measures

MRG: Refactor registration measures

Make it possible to switch off normalization of similarity measures used for image registration. Conventional similarity measures are normalized by default so as to be independent of the image overlap size, but this may sometimes not be desired.