Overwrite model parameter

[afard]

The main purpose of this PR is to resolve #605. Nevertheless, I made several other improvements as well.

• The overwrite_model parameter is moved from the session level config to a parameter of VerticaModel class
• It makes more sense to declare the _model_category property only in Supervised and Unsupervised classes. Then, their subclass will inherit the right one. In case of any exceptions, the property can be overriden.
• The model_name and overwrite_model are common attributes of all models so it makes sense to have them at the base class.
• The AutoDataPrep class needed its own drop function.
• Unfortunately, some tests didn't cleanup the database after themselves. The problem doesn't show up in the Github tests because each time a new database is created, but it is very annoying for a developer who runs the tests against the same database. I fixed those. Although there are still leftovers, but they don't cause test failures.
• I added the tests of the new overwrite_model parameter only for a few models. Nevertheless, it covers all if statements that have this parameter.