Hillmaker

Real reason is to deal with problems publishing to conda-forge.

- Added `edge_bins` parameter to control how occupancy contribution is computed
for arrival and departure bins. 1=fractional contribution (default), 2=whole bin

* Multiple category fields allowed.
* Can specify a field to use as occupancy weights. This can be useful
for building plots of workload instead of occupancy.
* Code updated to reflect deprecations and new functionality of pandas.

Alpha release. Basic hillmaker functionality in place. Usable from scripts and notebooks.