Request that Index provide a location?

[ctb]

In #1373, I added a location attribute to SBTs. We have something similar in LinearIndex and LCA_Database already (filename), and could add it to MultiIndex in #1374 as well.

The idea would be to provide a programmatic identifier for the location from which a database was loaded. In an ideal world, this would be something that the .load(...) function for that Index class could understand, although I don't want to require that just yet as it seems restrictive for in-memory Index classes.

In the case where a location was not available or did not make sense, it would be left as None.

See this comment on ZipFile for some thoughts on URL schemes to maybe use for this.

[ctb]

note that filename in the header for CSV output for search and gather should be changed to location for 5.0, if we do this.