Adds ComplexCoverageType.TEMPORAL_BROADCAST

Adds ComplexCoverage type 'TEMPORAL_BROADCAST' - combines multiple coverages along the temporal geometry of one (the first) coverage.  Temporal geometry for the other coverages are aligned with the nearest timestamp of the 'primary' coverage and then forward-broadcast until the next match is seen.
Adds test for TEMPORAL_BROADCAST

Adds test_temporal_broadcast
Fixes minor bug in TEMPORAL_BROADCAST

Addresses minor bug with how head_coverage_path was being reported in the TEMPORAL_BROADCAST ComplexCoverage type:  was None, should be that of the first reference coverage

Expands test_head_coverage_path
Merge pull request #77 from blazetopher/complex_broad

Temporal Broadcast ComplexCoverage