Make it clear which sectors are supported in a given version

[barentsen]

At present it is not clear exactly which sectors a given version of tess-locator supports.

I propose to resolve this as follows:

1. The version number will reflect the sectors supported, e.g. v30 will support sectors 1 through 30.
2. If a new version with additional sectors becomes available, users will receive a warning message upon import tess_locator, explaining which new sectors are available and how to upgrade.