Use Demeter irrigated land fraction by default

[crvernon]

Use Demeter’s outputs for irrigated and rainfed cropland for years 2010-2100 (5-yr intervals) for each of the 67,420 0.5 degree grid cells. This will replace having to apply the static fractions for 2005 that are currently in Tethys to all GCAM years.

We need to:

• Add this feature as the default option; keep existing method as an option
• Add functionality to process Demeter output into the format expected by Tethys
• Test against original