Open water stratum handling in composition based predictions

[psolymos]

We report average relative abundance at the 1km^2 pixel level for the landbase, which is the portion of the pixel not covered by water. In other words, 0 abundance for open water does not contribute to the pixel level mean. This is a feature and needs to be represented in the functions with sensible defaults.

Note: winter habitat use for winter-active mammals might render water as a use stratum.