Add a function to drop all-NaN columns in a bdb table

[curlette]

Running analysis with lovecat or cgpm fails on a table with any all-NaN columns. It would be nice to be able to quickly drop such columns automatically.

A helper function to do this would be great in part because of the limitation of SQLite to not allow columns to be dropped directly:

SQLite supports a limited subset of ALTER TABLE. The ALTER TABLE command in SQLite allows the user to rename a table or to add a new column to an existing table. It is not possible to rename a column, remove a column, or add or remove constraints from a table.