Refactor MetaValidator

[dc-almeida]

Closes #597
This PR includes the following:

• Refactors and expands MetaValidator
  • from_codelist: uses meta codelists to validate against allowed values
  • from_file: uses config files following a similar structure to DataValidator
    • Meta validation config files allow for a meta field that specifies the column(s) to validate, and name refers to the output column with validation results
  • apply follows a similar implementation to DataValidator, but for meta indicators
    • the value/values field allows specifying a list of values or a single numerical value. Lists are hard checks, single numbers are assessed with rtol/atol (bounds).
• Updates MetaCode, keeping a field alias for backwards compatibility
  • If backwards compatibility is not necessary, best to remove the alias, as it has a small caveat with Code extra_attributes
• Refactors DataValidator by extracting generic Validator and ValidatorCriteria parent classes from which meta and data validators inherit
  • this avoid code duplication and makes Refactor RequiredDataValidator to harmonise with DataValidator #603 easier (if relevant)
• Adds meta validation step in CLI command validate-project
• Updates tests
• Updates documentation