Add time-varying surface forcing to MOST

[ckendrick]

This adds options to SurfaceLayer to interpolate surface temperature and fluxes over time from a user-provided file. For the surface flux pathway, this is equivalent to using surface_layer.flux_type = "custom" but using time-varying data from a file instead of constant values.

The new options added are:

• erf.most.use_sfc_fluxes boolean flag to enable interpolating surface flux values from file.
• erf.most.use_sfc_sst boolean flag to enable interpolating surface temperature (SST) from file.
• erf.most.sfc_file path to text file, required if either option above is enabled.

The user-provided text file to use for the erf.most.sfc_file option has the following format:

day sst(K) H(W/m2) LE(W/m2) TAU(m2/s2)
 205.500   293.65      4.43     22.73      0.00

where each new line represents a distinct time value to interpolate between. The day column is converted to seconds and the elapsed simulation time in ERF is used.

For the SST pathway (erf.most.use_sfc_sst = true), the input-file format can be simplified to only day sst(K) column data, e,g:

day sst(K)
202.227083    290.460

Some configurations are not currently supported with these options (LSM, EB).