Add source sinks in bc format

[veenstrajelmer]

Relevant info:

• bc support for source-sinks will be implemented in the FM kernel in delft3d PR 1753 (issue UNST-5759)
• no changes should be required in HYDROLIB-core since this new implementation only requires some new QuantityUnitPairs
• an example of how to generate such an ext/bc is availabe in converters.py#L603
• an example of the QuantityUnitPairs is in converters.py#L861. This example generates 3 [Forcing] blocks, these can also be merged into one if desired.
• the documentation for ext file source sinks is available here: https://content.oss.deltares.nl/delft3d/D-Flow_FM_User_Manual.pdf#subsubsection.C.6.2.4.
• the documentation of bc-files for source sinks is still to be written: UNST-8687
• example mdu/ext/bc available in c010_sourcesink_2D.zip

Todo:

• add a function to dfm_tools that generates a bc file with source sinks and also updates the extfile, based on some arbitrary input variables (discharge, sal, tem, NO3, maybe vary constant/timeseries) and an x/y coordinate (or pli)
• print warning when calling the function that it only works from delft3dfm version so-and-so (date so-and-so), since this is very new stuff and most users don't have access to this version of delft3dfm yet
• add minimal example in the modelbuilder notebook that works anywhere globally (for now, disable by default because of the previous point)
• update whatsnew

Contributing guidelines:

• general contributing information available here (too much info, so cherry pick): https://deltares.github.io/dfm_tools/CONTRIBUTING.html
• workflow: create a branch from this issue, commit things, open a PR from that branch, request review when first iteration is done
• opening a PR also kicks off the testbanks on every commit, you can save CO2 by cancelling them if you are not interested in the result. Or wait with opening the PR until the branch contains a working proof of concept. Both are fine.
• keep track of code quality (analysed on each push to a PR by sonarcloud)
• make sure to cover new code with tests (analysed by codecov after every successful github pytest workflow run)
• after a PR is created, each push to the branch will trigger the pytest github workflows, these can be tracked here