Allow easier setting of multiple warning levels for same criterion in DataValidator

[dc-almeida]

IIRC, one of the suggested improvements was to change the write-up of multiple warning levels for the same validation criterion.
E.g.: currently, one would write:

- variable: Primary Energy
   year: 2010
   upper_bound: 5
   lower_bound: 1
- variable: Primary Energy
   year: 2010
   upper_bound: 2.5
   lower_bound: 1
   warning_level: low
- variable: Primary Energy|Coal
   year: 2010
   upper_bound: 5
   lower_bound: 1

This would mean "Primary Energy" would have two levels of warnings, error (by default) and low. But for that, the IamcDataFilter attributes (in this case, variable and year) have to be repeated for each level.

How would a better write-up look like? I leave an example below:

- variable: Primary Energy
  year: 2010
  warning_level:
    error:
      upper_bound: 5
      lower_bound: 1
    low:
      upper_bound: 2.5
      lower_bound: 1
- variable: Primary Energy|Coal
  year: 2010
  upper_bound: 5
  lower_bound: 1

In this case, I guess error could no longer be default, and would require quite a bit of refactoring.
Thoughts, @danielhuppmann @phackstock

[danielhuppmann]

I think this example makes sense. An alternative, as food for thought, would be more explicit

- variable: Primary Energy
  year: 2010
  validation:
    - warning_level: error
      upper_bound: 5
      lower_bound: 1
    - warning_level: low
       upper_bound: 2.5
       lower_bound: 1
- variable: Primary Energy|Coal
  year: 2010
  upper_bound: 5
  lower_bound: 1

This might be easier to implement because "warning-level" attribute doesn't have to be abused.