Add support for "Kc" variable in Collection methods (specifically .interpolate())

[cgmorton]

The interpolation code is currently hardcoded to work with "ETf" (fraction of reference ET) as the primary interpolation band (interpolate ETf to daily, multiply by daily ETr to get daily ET, then aggregate). To speed up development the interpolation code was not modified and an Image.etf method was added that returns kc. At some point it would be good to add real support for a user wanting to export "Kc" images or collections.