projection_source srs overwritten by dataset srs in reproject_raster_dataset

[kmuehlbauer]

reproject_raster_dataset is a nice function to reproject a GDAL raster dataset. Sometimes the projection information misses from the dataset, but can be supplied via projection_source kwarg.

Unfortunately it is overwritten unconditionally with the dataset projection, see

wradlib/wradlib/georef/raster.py

Lines 502 to 504 in b5a31e9

	if dst_srs:
	src_srs = osr.SpatialReference()
	src_srs.ImportFromWkt(src_ds.GetProjection())

This should only be done if projection_source=None. If projection_source is given it should not be overwritten.