Explicitly request h2i in RXCROPMATURITY gddgen phase

[samsrabin]

(Step 3 in parent issue #3319. Only necessary if Step 1, #3320, indicates this is a good idea.)

Add an explicit hist_avgflag_pertape(3) = 'I' in user_nl_clm for the GDD-Generating phase of RXCROPMATURITY tests (to avoid relying on the inheritance from the crop testdef), and add comments in various places explaining it.

Should make no difference in test results because it's already I by inheritance.

[samsrabin]

#3220 indicates that this is safe to do, so go ahead.