Retrieves a variable from its name. If multiple variables have the same name, this routine chooses one arbitrarily.
A non-zero return value indicates that a problem occurred while retrieving the variable. Refer to the Error Code table for a list of possible return values. Details on the error can be obtained by calling GRBgeterrormsg.
model: The model from which the variable should be retrieved.
name: The name of the desired variable.
varnumP: Variable number for a variable with the indicated name. Returns -1 if no matching name is found.