ENH: Add warning when loading old db

[adelavega]

To address loading incompatible saved dbs from previous various we could:

• Throw a warning when loading a db from a previous pybids version
• Catch SQL exceptions and suggest version incompatibilities as a potential reason for the error

Other ideas?

[effigies]

I would suggest that we add a database version, rather than the PyBIDS version. Call what we've had up to now version 1, move to an explicit version 2.