Allow for progress_cb_obj to be a callable or None

[roddehugo]

Check the progress object with PyCallable_Check which has the same signature than PyFunction_Check.

int PyFunction_Check(PyObject *o)
Return true if o is a function object (has type PyFunction_Type). The parameter must not be NULL.

vs

int PyCallable_Check(PyObject *o)
Determine if the object o is callable. Return 1 if the object is callable and 0 otherwise. This function always succeeds.

This commit closes #46