improve UGRID support

with this PR, we implement the ability to visualize variables on nodes and edges via the dual mesh. We base the UGRID decoder on the gridded package, particularly on https://github.com/NOAA-ORR-ERD/gridded/pull/61 and https://github.com/NOAA-ORR-ERD/gridded/pull/62

closes #29 closes https://github.com/psyplot/psy-maps/issues/32

• [ ] Tests added (for all bug fixes or enhancements)
• [ ] Tests passed (for all non-documentation changes)
• [ ] Fully documented, including CHANGELOG.rst for all changes

@platipodium, I'd highly appreciate if you test this implementation with your data. For me, everything works out of the box with the schout_5.nc file you sent me today.

Note that you have to install the gridded package from https://github.com/NOAA-ORR-ERD/gridded/pull/62 and this branch. You can do this for instance via

pip install -U git+https://github.com/Chilipp/gridded@dual_node_mesh git+https://github.com/Chilipp/psyplot@ugrid 

Works for me regarding both issues

• start_index
• dual mesh

Other things are still not perfect, but should go to a different bug report. Thanks for fixing these issues so quickly.

thanks a lot @platipodium! I need to wait for the decisions in the gridded PRs, but I hope they will come soon

closing this PR, see https://codebase.helmholtz.cloud/psyplot/psyplot/-/merge_requests/31