The default of 1e-13 was letting through some state updates that look
like they should have been truncated to 0: each time step, snocan was
decreasing by about 12 orders of magnitude.

Addresses ESCOMP#2444

Explicit A/C adoption

Code changes for adding an explicit air-conditioning (AC) adoption parameterization scheme in CLMU. This includes adding a new time-varying input variable (AC adoption rate, p_ac), changes to building energy calculations, and a toggle (new namelist variable urban_explicit_ac).

In this tag we keep the change off by default in order to show that answers do not change.

Fixes ESCOMP#2254 Explicitly representing air-conditioning adoption in CESM.

slevis resoloved conflicts:
doc/ChangeLog
doc/ChangeSum