Revise treatment of sub-grid hydrological fractional areas

[billsacks]

From @martynpclark on August 24, 2017 23:16

The current representation of sub-grid fractions in CTSM may be inconsistent/confusing. Revise this code to improve clarity and correctness.

Copied from original issue: NCAR/clm-ctsm#6

[billsacks]

I think this is referring to the subgrid fraction that is covered with ponded water, etc. Revising the title accordingly.