new GRASS virtual raster in trunk

A new GRASS virtual raster format has been added to trunk in r72761-4,
together with a new module to build a GRASS VRT: r.buildvrt.

*r.buildvrt* builds a virtual raster (VRT) that is a mosaic of the list of
input raster maps. The purpose of such a VRT is to provide fast access to
small subsets of the VRT, also with multiple simultaneous read requests.

*r.buildvrt* creates a list of raster maps that can be located in different
mapsets. The ouput is a read-only link to the original raster maps which is
only valid if the original raster maps remain in the originally indicated
mapset. A VRT can also be built from raster maps registered with
*r.external*.

Reading the whole VRT is slower than reading the equivalent single raster
map. Only reading small parts of the VRT provides a performance benefit.
Please test!

Markus M

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