Introduce support for gzip databases

[rbiswas4]

The precomputed HealpixelizedOpSim stored for NSIDE=128 (NSIDE=256 is too large ~ 1GB) corresponding to the enigma_1189_micro.db database used for continuous integration is ~200MB. This can be brought down to ~50 MB using gzip compression as shown in scripts/make_healpixeldb.py. While ~200MB will probably work, it is still a lot of space, and it would be good to replace this with the ~50 MB file. This also means that tests will have to start from gzipped opsim databases.

• include gzipped version and add script in .travis installs to unzip the database in situ
• Improve support by using sqlalchemy + gzip libraries to make opsim_output.py be able to do this.

[rbiswas4]

Given the improvements in #101 , These may be unnecessary. Will close after this is conformed by #102