Add warning-feature to DataValidator

[danielhuppmann]

As a next step for scenario validation and vetting, we need to extend the DataValidator class to enable showing a warning instead of raising an error.

My proposed implementation is the following:

• Add a warning-level attribute to a validation-criteria item, which defaults to None, but can be set to e.g. "red", "yellow"

• This will probably require an intermediate class DataValidationCriteria(IamcDataFilter) with the new attribute, and DataValidationCriteriaValue and ** DataValidationCriteriaBounds** inherit from that.

• If the validation fails and the warning-level is not None, add the value to a warning-list in
  

  nomenclature/nomenclature/processor/data_validator.py

  Line 96 in 3a76cba

  for item in self.criteria_items:

  instead of the error-list.

• If the warning-list is not empty, display the warning-list as a DataFrame (after the error-list, but before raising the error).

Does that make sense, @phackstock?