Add ladder-style solid attenuator transmission calculation tools + function blocks

[klauer]

Initial implementation is here: https://github.com/pcdshub/lcls-plc-kfe-rix-motion/pull/40

• May require some generalizing / additional documentation.
• May require new dependency: lcls-twincat-physics for calculations
• Plan is to test it on AT2K2 first, merge in the final version here, then move on to AT1K4, AT2L0, ...