Add three functions for variables

[pfetsch]

This pull request adds three function concerning variables:

• vstatus for obtaining the variable status;
• isActive to check whether a variable is active;
• markDoNotMultaggrVar to mark a variable to not allowed to be multi-aggregated.

The functions are needed in a secondary project.

[Joao-Dionisio]

Thank you @pfetsch ! Can you please also add SCIPmarkDoNotAggrVar() and a basic test to the added methods and variable type?

[pfetsch]

Thank you @pfetsch ! Can you please also add SCIPmarkDoNotAggrVar() and a basic test to the added methods and variable type?

Yes, will do. This will take a while ...

[Joao-Dionisio]

@pfetsch I can do it, no problem. Thank you for the contribution!