Be stricter about vtype values

[simonbowly]

Here's one that's a bit more problematic:

x = gppd.add_vars(model, df, lb="lb", ub="ub", vtype="notmytype")

whether or not "notmytype" exists as a column, this creates semi-integer variables because gurobipy is very permissive and just takes the first letter of the string ...

Originally posted by @simonbowly in #91 (comment)

[simonbowly]

We should be stricter to avoid confusion, irrespective of whether we implement #91.