Added LYRA time series utilities for downloading and parsing the LYRA Timeline Annotation File (LYTAF)

Added some utilities to lyra.py to allow for downloading and querying of the LYRA Timeline Annotation File (LYTAF). After some discussion, these utilities were placed outside of the lyra lightcurve class. The added routines are:

 - download_lytaf_database
     downloads the full LYTAF pointing database to a chosen location. The database is a sqlite file.

 - get_lytaf_events
     queries the LYTAF sqlite database to find pointing events that occurred within a given time range

 - lytaf_event2string
     a short utility to convert LYRA event integer descriptors to strings.