Converter for old external forcings file

[arthurvd]

Is your feature request related to a problem? Please describe.
D-Flow FM has an "old external forcings file" (ExtOldModel in HYDROLIB-core), which is about to be phased out and replaced by blocks in:

• new external forcings file (ExtModel in HYDROLIB-core)
• structures file (StructureModel in HYDROLIB-core))
• initial fields and parameters file (IniFieldModel in HYDROLIB-core))

We would like a converter that takes an *.mdu file as input, scans for any old ext file, and exports a new MDU file, with underlying new attribute files as listed above.

Describe the solution you'd like
A script, both callable as a function, as well as from the commandline.

Additional context
See D-Flow FM kernel issue UNST-7872 for the actual migration epic.