`fetchNASIS()` and `fetchVegdata()`: conditionally exclude extended tables in queries

[brownag]

This would allow for reduced sets of tables to be used in database snapshots by checking presence of "extended" tables in the database prior to running queries (which result in errors if tables do not exist)

Via an argument user could specify a character vector of extended tables to include, or FALSE to exclude all extended data.

Warnings would be issued in the event tables are missing, unless the user explicitly is setting the argument in such a way that they should be excluded.

[brownag]

It may be easier to do this with a charcter vector of tables to exclude and a default TRUE to include all extended data

[smroecker]

The way I was thinking about implementing this function was by having the function return NULL if a table was missing. That way it could adapt seamlessly to different database connections which might only be partially complete. I realize this might require some tinkering.