create functions to translate NDTI --> tillage modifier

[dlebauer]

Goal --> translate $\delta$NDTI to SIPNET decomposition rate modifiers

Approach:

1. [Model agnostic function] use published mappings of NDTI --> tillage intensity categories, with linear or spline interpolation through category midpoints. [This could be model-independent]

• inputs: date, delta NDTI
• outputs: date, tillage intensity

2. [SIPNET specific function] that interpolates from tillage intensity --> decomposition modifier

• inputs: date, tillage intensity
• outputs: date, decomposition rate modifier