Separate namelist validation inConfigIconTask to dedicated namelist class

[agoscinski]

Is PR #68 we introduce the field validator for namelist into ConfigIconTask. The reason why we cannot put it in separate class with its own validator but have to iterate here is described here

Sirocco/src/sirocco/parsing/_yaml_data_models.py

Lines 393 to 395 in 3e4da24

	# validation done here and not in ConfigNamelist so that we can still
	# import ConfigIconTaskSpecs in core._tasks.IconTask. Hence the iteration
	# over the namelists that could be avoided with a more raw pydantic design

However this might be fixable but it is for the moment too time consuming what the right way is to do as pydantic is complex due to its flexibility.

[leclairm]

Could be solved by a PR addressing #88. Otherwise do it after closing #88.