Create composite index on (mjd, healpixel, obscode)

[moeyensj]

Also delete single indices and add index on dataset_id in datasets table. Closes #40.

PR is currently being tested on the NSC dataset, will report back once complete.

[moeyensj]

I tested this on a small sample of the NSC dataset (~4 hdf5 files indexed using this branch).

import sqlite3 as sql

con = sql.connect("ndet4_test_00/index.db")
curs = con.execute("""SELECT name FROM sqlite_master WHERE type = 'index';""")
result = curs.fetchall()
print(curs.fetchall())

Which returns: [('fast_query',), ('sqlite_autoindex_datasets_1',), ('ix_datasets_id',)]

So it looks like the composite index is successfully created when the database is first indexed.

[ntellis]

I've tested this, and it's working perfectly.

My only thought is: does it make sense to check for this index when loading a database, ex in PrecoveryDatabase.from_dir? Because the performance really tanks without the index.

[moeyensj]

That's a good idea. Basically, check for the index and see if it exists and if it doesn't then throw a warning?

[moeyensj]

@ntellis I've added a warning when the index can't be found.