Incorporate Terra (3rd order) shims into numerical model for "reference map"/basis

[rtopfer]

The current code for modeling Siemens shim fields only covers 1st and 2nd order terms (based on the Prisma). Ideally, it would also cover the 3rd order terms on the Terra.

This requires, first, checking the Adjustments/Shim card on the Terra console to see what the corresponding shim terms and values actually are, and then updating the code accordingly.