Communicating land heterogeneity via CLUBB

As part of the CLASP CPT, we're looking into communicating land subgrid heterogeneity up to the atmosphere via CLUBB's surface higher order moments. That means computing the higher order moments in the land model, passing them through to CAM, and adding an option to either use those moments computed in CLM for the surface or maintain the default behavior of computing them in CLUBB. We'll also need to add some interface to handle the case where the CLM moments are used.