Add type identifiers to scip.pxi

[Joao-Dionisio]

Using branchVar(v) will crash the code if v is not a scip variable.

A simple fix for this is to change the definition in scip.pxi from

def branchVar(self, variable):

to

def branchVar(self, Variable variable):

Plenty of other examples exist in the code. Besides making the code safer, it will also make it (slightly) faster.

EDIT: Adding types to loop variables is also desirable, given that it allows Cython to convert Python loops into C loops. See Issue #750 for more information. EDIT: This is now done, in PR #758.