Omitting `dataset_inputs` prints `experiments` missing

[siranipour]

If I purposefully forget the dataset_inputs in the runcard:

fit: NNPDF31_nnlo_as_0118

use_cuts: fromfit

theoryid: 53

pdf:
  from_: fit

template_text: |
  {@groups_chi2_table@}

actions_:
 - report(main=True)

I get the duplicated error message below:

[ERROR]: A parameter is required: experiments.
This is needed to process:
 - group_dataset_inputs_by_metadata
through:
 - ()
through:
 - report
through:
 - template_text
through:
 - groups_chi2_table
through:
 - groups_data

[ERROR]: Bad configuration encountered:
A parameter is required: dataset_inputs.
This is needed to process:
 - group_dataset_inputs_by_metadata
through:
 - ()
through:
 - report
through:
 - template_text
through:
 - groups_chi2_table
through:
 - groups_data

possibly because of some self.parse_from_ craziness within config.py

[Zaharid]

We discussed this at the time, but I seem to recall there wasn't an obvious fix. Would be good if you could have a more detailed look. That said, after 4.0 is released, I would suggest removing experiments altogether.