[pnm][griddata] Fix griddata reader when using non-native CRLF file ending on Linux
Handle a bug when a user uses a dgf file with non-native CRLF file endings (Windows). We might not have to deal with this actually but it's better not to crash the program because of it. The Dune DGF reader seems to handle CRLF fine.
Also improve the error message in case the header is not as expected
Edited by Timo Koch