Find active cells with idomain > 0 instead of idomain == 1

[JoerivanEngelen]

The LHM apparently has > 1000 cells where IDOMAIN == 2. I noticed that wells located in these cells get masked away. For both iMOD5 as well as MODFLOW 6, IDOMAIN > 0 means active cell. iMOD Python, however, enforces/assumes IDOMAIN == 1, but we don't actually validate this.

I think it is more user friendly to support IDOMAIN > 0. A quick search showed that we only have a few files where active is computed as idomain == 1, so it appears to be low hanging fruit.