New export options argument added to bfmSpatial

'returnLayers' specifies which parameters of the bfm result to return. Each parameter is a layer of the resulting raster brick.

This is working on a very small test subset, but error catching still needs to be tested.
Further testing and updated documentation.

As far as I can tell, the added returned values do not slow down the function at all.
updated bfmSpatial documentation
merge develop into feature-bfmSpatial
roxygenised everything with roxygen2 version 4.0.2

...which changes the header on each of the .Rd files
merge feature back into develop