Add weighted voting pool support

[Pablete1234]

Adds support for map-weighting in voted pools, as well as allowing more flexibility in defining maps in groups in the map-pool config file.

Currently, the pool has simply one maps list of strings:

pools:
  default:
    [...]
    maps:
    - Airship Battle
    - Harb
    - Race for Victory
    - The Fenland
    - Warlock

This remains supported, but can now (for all types of pools, not just voted) be extended to be grouped, eg:

pools:
  default:
    [...]
    maps:
      dtc:
        - Airship Battle
      tdm:
        - Harb
      ctw:
        - Race for Victory
      dtm:
        - The Fenland
        - Warlock

These groups, can also be weighted (you can do this for all pools, but weight will have no effect on non-voted pools):

pools:
  default:
    [...]
    maps:
      dtc:
        - Airship Battle
      tdm:
        - Harb
      ctw:
        - Race for Victory
      dtm:
        weight: 0.5
        maps:
          - The Fenland
          - Warlock

Note that you can mix-and-match the above, and even recursively nest stuff:

pools:
  default:
    [...]
    weight: 0.5
    maps:
      dtc:
        weight: 1
        maps:
          - Airship Battle # weight = 1
      tdm:
        - Harb # weight = 0.5
      ctw:
        - Race for Victory # weight = 0.5
      dtm:
        weight: 0.7
        maps:
          s-tier:
            - The Fenland # weight = 0.7
          overplayed:
            weight: 0.2
            maps:
              - Warlock # weight = 0.2