Add ability to specify sample normal in physics package

[psavery]

We need to think of exactly how we want the user to input this, because the hexrd convention for the sample normal might not be intuitive to them.

Once we have it, we'll use it (along with the current XRS) to compute rMat_s which will be input here. This is needed to compute the transmissions properly.

We may end up needing the sample normal elsewhere, too.