Remove Response Function

[rodriguez-jay]

When assigning multigroup adjoint sources we have required the creation of a ResponseFunction as follows:

def ResponseFunction(xyz, mat_id):
    response = xs_det.sigma_a
    return response
response_func = VectorSpatialFunction(ResponseFunction)
adj_src = VolumetricSource(logical_volume=det, func=response_func)

The same source distribution can be assigned in a similar fashion to what is already done for forward sources:

Q_adj = xs_det.sigma_a.tolist()
adj_src = VolumetricSource(logical_volume=det, group_strength=Q_adj)

I believe the requirement for using a response function is an artifact from lua and no longer necessary.

[ragusa]

LEt's talk with @wdhawkins on Monday but I think this can go (it was likely done this way because of lua)