Add example to convert an existing 'vanilla' Modflow 2005 model

[Joeri van Engelen]

People who want to transition to the iMOD Suite might have an existing Modflow 2005 model, the 'vanilla' USGS version, that is.

This model can be read using Flopy and consequently be written to a iMODFLOW or Modflow6 model using iMOD Python.
An example of this in either the iMOD Suite documentation or the iMOD Python documentation, would be very useful.

This might be something for the iMOD Python documentation.