User's Guide: Add section on crop postprocessing

[samsrabin]

Crop postprocessing often trips people up. We should provide instructions in the User's Guide and link to those instructions in the Tech Note.

Definitely include:

• If you care about crops, you will probably want to save outputs at the PFT level. Provide instructions for doing this, including why you might want to do this on a separate history tape from other outputs (PFT-level outputs are large and hard to work with).
• Some crop-related history variables include a per-sowing (mxsowings) or per-harvest (mxharvests) dimension, which can be non-intuitive. See this forum post for an example. The purpose and utility of these should be explained.

Maybe include in first go at this issue:

• Provide a script to calculate annual area, production, and yield. Work with regular, per-harvest, and per-year values. Optionally combine irrigated+rainfed, tropical+temperate, spring+winter; if doing this, explain which CFTs got combined into which crops. Optionally provide paramfile to add, in a global comment attribute, which non-simulated crops correspond to which simulated ones. This script would need to be tested for no errors in aux_clm.

Do this eventually (file as separate issue(s)):

• Provide a script to align time series according to planting year. Something to consider: For a crop planted in one year and harvested the next, which year's area should be used?
• Explain about the crop CUPiD notebook.