GSF Horner method for polynomial evaluation "un-roll" up to order 5

In GSF we use the Horner method for polynomial evaluation (the ones parametrizing the energy loss) . Here we "un-roll" up to order 5 .

In reality that is the max order we have in practice , but for now leave a fallback.

In addition we add a unit test (testing a "known" function) for this as we had none before.