Add subsetRasterTS() to functions where 'sensor' is used to subset

[bendv]

No description provided.

[bendv]

TODO: check whether subsetting the raster a priori is more efficient than doing this on a per-pixel basis inside mc.calc()

[bendv]

implemented at pixel level rather than subsetting rasters first. This function can be left as an optional pre-processing step, but doesn't need to be called in other functions like bfmPixel() and bfmSpatial()