[Utilities] add support for VariableIndex in ModelFilter

[odow]

Closes #2308. But I don't really like this. I think it's better just to delete variables after the copy---that will ensure we delete the appropriate constraint coefficients, etc.

[odow]

The more I think about this, the less I like it. Deletion is the way to go.

[blegat]

The issue is that it's also unclear what to do with constraints that have these variables, remove them or replace the variable with zero ?
In the first case, we could have to alter ListOfConstraintIndices as well but we don't want to do that unless we know the filter is removing variables. So I'd add an option specifying SET_VARIABLE_TO_ZERO, REMOVE_CONSTRAINTS_WITH_VARIABLE (in that case, it cannot be in the objective) or KEEP_ALL_VARIABLES.
By default, we have KEEP_ALL_VARIABLES and we error if the filter remove any variables from the list (to let the user know he also need to specify that option).

[odow]

The issue is that it's also unclear what to do with constraints that have these variables, remove them or replace the variable with zero ?

Yes. But there are just too many complications. We don't need to support this because the user can just delete variables after the fact.