Fix switchgrass and miscanthus distributions

[teixeirak]

Switchgrass and Miscanthus distributions are derived from a map ‘DayCentFinalYield(5).tif’ (for Miscanthus, not sure of map name for switchgrass). The Miscanthus biome (and probably switchgrass) shows up wherever there is data, even for the large number of sites with yield << 1 Mg/ha. Change so that it only shows up above a threshold yield of 3 Mg/ha (somewhat arbitrary, based on discussions with Evan DeLucia & David LeBauer).

[teixeirak]

My statement in the previous comment appears to be incorrect. Based on these lines, the potential range of switchgrass and miscanthus is defined simply by where the map file is present. This leads to them showing up as options in inappropriate places.

We have modeled switchgrass and miscanthus yields in this folder: https://github.com/ebimodeling/ghgvc/tree/master/netcdf/GCS. The citation for these is Davis et al. 2011. To fix the miscanthus and switchgrass distributions, we will need to load these maps and list these as potential ecosystem types only when yield ≥3 Mg/ha.

[teixeirak]

Closing for now, as these ecosystems are not included in the current version. See issue #125.