short-circuit for sampleRasterStackByMU() when number of pixels is low

[dylanbeaudette]

There should be a that indicates whether or not to use samples or all of the pixels within a polygon. For example:

• 10m slope map: n_.samples << n_.pixels → use samples
• 800m PRISM: n_.samples >> n_.pixels → use all pixels within polygon

Right now, sampling is done in this context:

# extract raster data, sample ID, polygon ID to DF
      l.mu[[mu.i]] <- rapply(raster.list, how = 'replace', f=function(r) {
        res <- data.frame(value=raster::extract(r, s), pID=s$pID, sid=s$sid)
        return(res)
      })

Alternatively, we can make the change earlier in the algorithm before constantDensitySampling() as a way to save some time. The short-circuit would use the pixel centers as the sampling points so that all following code could remain the same.