[Bug] - Confusing warning message when loading messy GEN files

[JoerivanEngelen]

Bug description
I'm loading messy GEN files from IBRAHYM, but it results in an incorrect warning:

UserWarning: Inconsistent IPF: header states 2 columns, first line contains 3

This stems from the imod.formats.ipf._infer_delimwhitespace helper function from the IPF module being used in the GEN module.

• The following needs to be done: create a small imod.formats.common module where useful helper functions can be shared
• Add an argument so that the warning message text "IPF" or "GEN" can be set.