rpc_from_geotiff fails if there is a textual RPC.txt next to the tiff

[AmitAronovitch]

It appears that if GDAL sees next to a geotiff my_filename.tif a file named my_filename_RPC.txt, then it will read the RPC tags from it rather than the "RPC" tag that is stored inside the tiff...

The problem is that the textual files (ikonos format) contain units, which rpc_from_geotiff does not expect. So it fails with

E           ValueError: could not convert string to float: '+005124.00 pixels'

This PR includes a regression test that demonstrates the problem, as well as a fix