Add DetectorFactory to build dynamically defined detectors.
Add WatchmanDetectorFactory to generate alternate pmt coverages on the
fly.
covering_fraction -> photocathode_coverage, and calculate pmt_area from
specified pmt_type.
Update docs to reflect addition of GeoFactory
Specity veto pmt coverange for WatchmanDetectorFactory with
DETECTOR.veto_coverage
Merge pull request #9 from BenLand100/detectorfactory

Add DetectorFactory and dynamic specification of Watchman coverage