[FEATURE] - find successor for ISG format

[FransRoelofsen]

ISG is the iMOD5 format (vector based) to store relevant information for surface water elements.
It is a Deltares format and this issue is to find a file format that is more in line with international standards.

The first ideas for this format / data structure are already shares in a related issue: #326.

The ISG topic is split in these 2 issues:

• this issue to define the new format and make it possible to create surface water elements from scratch
• the related issue is there to add functionality in case a reader for the iMOD5 ISG format is also needed.

[Huite]

this issue to define the new format and make it possible to create surface water elements from scratch

This should be straightforward: simply draw the line features in QGIS, or use existing vector geometries (and possibly simplify them).
A UGRID1D topology and Dataset can be created directly from using xugrid's from_geodataframe() converters.