New ``PyUFuncObject`` field ``process_core_dims_func``

The field ``process_core_dims_func`` was added to the structure

``PyUFuncObject``. For generalized ufuncs, this field can be set to a

function of type ``PyUFunc_ProcessCoreDimsFunc`` that will be called when the

ufunc is called. It allows the ufunc author to check that core dimensions

satisfy additional constraints, and to set output core dimension sizes if they

have not been provided.