FATES_PATCHAREA_LU and FATES_PATCHAREA_AP are not in units of m2, but rather m2/m2. It is thus misleading for them to be named "AREA". In the interest of not messing up user workflows, I'm not changing those---at least for now. However, this commit renames related internal variables in the history code for clarity.

Specifically, "area" is replaced with "fracarea" (fractional area, relative to site area) in the following:
- ih_area_si_landuse
- ih_area_si_age
- hio_area_si_landuse
- hio_fracarea_si_age

1) Makes it clearer that this is a patch-level variable.
2) Uses "fracarea" instead of "area" for clarity and consistency with previous commit.

- FATES_ZSTAR_AP
- FATES_SECONDAREA_ANTHRODIST_AP
- FATES_SECONDAREA_DIST_AP
- FATES_BURNFRAC_AP
- FATES_FIRE_INTENSITY_BURNFRAC_AP
- FATES_VEGC_AP
- FATES_FUEL_AMOUNT_AP
- FATES_CANOPYAREA_AP

Using fates_patch_type%area_by_age allows consolidation into one block, rather than splitting the calculation across two parts of update_history_hifrq2.

Moving calculation of canopy_area_by_age to an earlier patch loop allows consolidation into one block, rather than splitting the calculation across two parts of update_history_hifrq2.

Instead of dividing by total site area, divide by age class area.

…s area, not site area.

Should fix FATES_MORTALITY_CANOPY_SZAP and FATES_MORTALITY_USTORY_SZAP.