NF: add and test TimeConvolver class

TimeConvolver wraps a function of time, a support interval and a time
delta to make it simpler and more efficient to convolve time courses.
TST: add use of TimeConvolver for HRFs

Test TimeConvolver again aganst convolve_functions, and summing of
individual convolved events.
RF: small python3 preparations

Small changes getting ready for migration to Python 3 common codebase.