Add `gdalWarp` option to `GDALDataset` constructor

[jjrob]

osgeo.gdal.Warp() projects a GDAL raster dataset object. By passing format='VRT' to it, you can get a GDAL virtual dataset (VRT) back. MGET's GDALDataset class should gain a gdalWarp option to take advantage of this. This would allow MGET to project a GDAL raster dataset to a new coordinate system on the fly when data are read by calling osgeo.gdal.Warp() and wrapping the resulting virtual dataset.

The description of gdalWarp would be something like this:

dict instance giving options to pass to gdal.Warp(). If provided, gdal.Warp() will be called on the dataset after it is opened and used to create a GDAL virtual dataset (VRT), and this GDALDataset instance will wrap the warped virtual dataset instead. If None is provided, gdal.Warp() will not be called.