feat: external forcings converter

[arthurvd]

Closes #621

[arthurvd]

@priscavdsluis / @tim-vd-aardweg : what do you guys think about this extra "legacy.py" inside the MDU directory?
As long as we don't have versioned support for several old/newer formats of some of our filetypes, this was the solution I came up with.

[tim-vd-aardweg]

@arthurvd / @priscavdsluis
I am not entirely sure to be honest. My first question is: Do we want tools in HYDROLIB-core, or do we create a separate package for tools that use HYDROLIB-core?

But let's assume we do want to add tools to the HYDROLIB-core package. I think my next question is: Should we implement this 'legacy' model into hydrolib, or should it be defined in the tool that uses it? Also, the change you request touches on a broader topic that should be discussed: How do we handle old/deprecated files/sections/keywords. Your requested change is due to the fact that we cannot handle older/unexpected file formats. And your change fixes things for your specific use case.

Having said all that, my thoughts are as follows:

• We will need to generically support older/unexpected formats. That way, we don't need your specific legacy model.
• Since that probably won't happen in the short term, we need a different solution for your use case. My preference would be to add the LegacyModel to the specific tool, so we do not pollute the main HYDROLIB-core package with quick fixes for specific tools.

[MRVermeulenDeltares]

Hi @arthurvd / @tim-vd-aardweg / @priscavdsluis , I know you didn't ask my opinion but I still want to ask some questions regarding this.
Previously I remember a presentation was given with a picture showing hydrolib-core and hydrolib.
And how it should be used by tools.
See https://www.deltares.nl/expertise/projecten/hydrolib-gezamenlijke-automatisering-op-watersysteemmodellen

Should the converter not be a tool build on top of hydrolib-core?
Especially if the support for the old file format is dropped and no longer backwards compatible supported by the kernels.
Should hydrolib-core still support the old format, or only the new format from version x?

[sonarcloud]

Quality Gate failed

Failed conditions
5.7% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[RStolkerDeltares]

@arthurvd I'm trying to run the converter script through the command line but it's failing with the error message below.
I did install the poetry environment as described in the docs.

poetry run python main_converter.py

Traceback (most recent call last):
  File "main_converter.py", line 35, in <module>
    from .converter_factory import ConverterFactory
ImportError: attempted relative import with no known parent package