xmax and ymax should be removed from irap_header

They can always be computed from the the other information in the header. Makes more sense to do this on export instead of making it possible to use the wrong xmax/ymax.

They should always be:
```
header.xmax = header.xori + (header.ncol - 1) * header.xinc;
header.ymax = header.yori + (header.nrow - 1) * header.yinc;
```

Start by locating the irap_header definition and the export path in the repository. Check how xmax and ymax are currently stored or written, then verify that export derives them from xori, yori, ncol, nrow, xinc, and yinc without allowing inconsistent header values.

Written by the indexing model from the issue text.