alter gips.tiles.Tiles.mosaic to handle raster derived SpatialExtent

[ircwaves]

In the case of rasters, perhaps we would want to make a utils.gridded_export that takes the site_mask and a list of rasters, and warps them into an all-no-data file (created from the site mask), and then masks the image.

man gdalwarp skrev:

 Mosaicing into an existing output file is supported if the output file
 already exists. The spatial extent of the existing file will not be
 modified to accommodate new data, so you may have to remove it in that
 case, or use the -overwrite option.

[justinfisk]

created utils.gridded_mosaic and hooked up to Tiles.mosaic using the above plan.

Probably needs a way to specify sampling method and possibly other options to gdalwarp

committed in 2add7d9