Avoids deepcopy for readonly coverages

Huge performance improvement across the board.
Merge pull request #131 from ooici/deepcopy

Avoids deepcopy for readonly coverages
Committing for collaboration with Jim Case.
Implemented conversion from HDF parameter groups and RTreeProxy to db backed parameter groups and RTreeProxy.
Adds check for Cassandra to HDF-specific file tests.
Adds test to verify cache locks

Note: this test took forever for me to figure out how to synchronize between multiprocessing and gevent, not fun.
Fixes issue with cached locks and cache invalidation
Initial release.
Merge branch 'hdf2db_testing' of https://github.com/oceanzus/coverage-model into HDFtoDB
Error handling for empty requests

Originally slices where the start equals the stop or the start exceeds the extents then an IndexError
is raised through the stack resulting in problems reading from the coverage. This commit adds error handling in the
get_value_dict logic to return empty arrays in these two cases instead of raising an error.
Merge pull request #133 from ooici/locking

OOIION-1497 HDF File Locking Cache
Merge pull request #135 from ooici/empties

OOIION-1502 Error handling for empty requests
Initial implementation of Postgres-backed metadata.
Merge branch 'master' of https://github.com/ooici/coverage-model into HDFtoDB