DOC+TST: test exclude keyword, document parcels

Using 'exclude' not previously tested.  Rewrite docstring for parcels.
Change exclude argument to be tuple to avoid mutable default argument.
force each run to the same affine in realign4d
making realign truly consistent with runs with different affines
add test
ds105 fmri example with single runs being fit
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
Merge pull request #253 from alexis-roche/fix-multisession-realign

extend realign4d to deal with runs with different affines
modifs suggested by Matthew
fix clumsy test