convert_grid_format triggers UserWarning in xtgeo

convert_grid_format (which shows up as multiple forward model steps in Ert, ECLINIT2ROFF, ECLRST2ROFF, etc) will (often?) trigger a UserWarning from xtgeo:

`UserWarning: Axis units specification is missing in input, assuming that no unit conversion is necessary
`            
coming from:
https://github.com/equinor/xtgeo/blob/main/src/xtgeo/grid3d/_ecl_grid.py#L541-L545