Eliminated fmapper/util.py replacing it with alternatives

wrap was either replaced with modulo or moved into the file where it
is used, i.e. fmapper/distribution.py.
frange was replaced with np.linspace, which meant switching from
increment based feature value ranges to number of steps.
cross_product replaced with itertools.product
Moved fmapper/core.py into fmapper/__init__.py eliminating core.py