Rename _var to _arg in MipWrapper_neg and abs to avoid name clash with MipWrapper_Expression.
Fix for seg fault when calling get_value on a MIP variable that did not need to be added to the model.

The variable may not be passed to the solver if it is not involved in a non-trivial constraint. get_value() will return the variable's lower bound as the value in this case.
Doc additions to clarify the behaviour of get_value() in cases where the variable is not passed to the solver.