Unify meta/settings files (e.g. times_mapping.txt and times-info.json)

[siddharth-krishna]

Here's a proposal for how the combined mapping/category file could look. For the mapping G_DRATE[REG,DATAYEAR,CUR,VALUE] = ~FI_T(Region,Year,Curr,VALUE,Attribute:G_DRATE), we could have the entry:

- name: G_DRATE
  category: parameter
  times-cols:
    - REG
    - DATAYEAR
    - CUR
    - VALUE
  xl-cols:
    - Region
    - Year
    - Curr
    - VALUE
  filter-attribute: G_DRATE

Alternatively, in JSON format (though perhaps YAML is a bit more readable? YAML also allows comments)

[
  {
    "name": "G_DRATE",
    "category": "parameter",
    "times-cols": [
      "REG",
      "DATAYEAR",
      "CUR",
      "VALUE"
    ],
    "xl-cols": [
      "Region",
      "Year",
      "Curr",
      "VALUE"
    ],
    "filter-attribute": "G_DRATE"
  }
]

A benefit of having a unified file is that it is less likely that an entity will be in one and not another, a bug that we've seen e.g. in:
#102 (comment) (cc @olejandro )

[olejandro]

Ideally time-info should already have priority over the mapping file, i.e. we should only use the mapping file for the entities present in it. But let's not do anything about it yet, until we are clear on the way forward?

[olejandro]

@siddharth-krishna let's start by utilising time-info where it is straightforward to do so already? E.g. we can already use it to specify a list of legal attribute names (UC_*) in process_user_constraint_tables.

[siddharth-krishna]

We should also consider using the TOML format, which many Python projects have been adopting as a replacement for JSON and YAML files: https://realpython.com/python-toml/