Fixes bug with temporal_interleave

Fixes issue with temporal_interleave allowing any number of coverages to be interleaved - was previously restricted to 2 coverages
Fixes bug when querying an empty coverage

Addresses an error that was being raised when querying an empty coverage.  Should now return an empty array if the coverage is empty.

Updates tests for empty coverage
Merge pull request #72 from blazetopher/inter_3plus

Fixes bug with: temporal_interleave & empty coverage data query