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