normalize crown area weighted height history diagnostic

[rgknox]

The calculation of crown area weighted canopy height is not complete. It needs to be normalized by total crown area of all cohorts participating in the site level sum. Here is where we accumulate the mean height:

https://github.com/NGEET/fates/blob/sci.1.72.1_api.33.0.0/main/FatesHistoryInterfaceMod.F90#L3604-L3605

There needs to be a sum of crown area, and division through by that area, following the cohort/patch loop.