revamped lower-level version of cython fill_sinks()
efficiency improvements to fill_sinks

the Python-version of fill_sinks now uses heapq and is about 20% faster

the cython-version has been completely rewritten, and is now about twice as fast
as the python version

rearranged module structure
add rfuncs module (equiv to crfuncs.pyx)
fix raster/__init__.py regression related to fill_sinks revamp