document instance database and make it customizable

[svigerske]

constants.py defines a location for a solu database:

rubberband/rubberband/constants.py

Line 11 in c7f3545

ALL_SOLU = FILES_DIR + "instancedata/database/instancedb.sqlite3"

Evaluation will fail if this is not available. The README.md does not mention that this needs to be provided and what the database schema is.

In addition, the location should be customizable via the configuration file (app.cfg).
Ideally, a remote location, or a webrequest, can be specified that Rubberband can query to get the latest version of the database.