Gameweek dependent free transfer value list

[mloujapero726381837]

Extends ft_value_list to support being a dictionary of gameweek ranges, while preserving original syntax. With this, it is possible to specify, for example, free transfer values that only apply for a subset of all weeks, but not without specifying a base list, which is to be applied when no range is applicable for a given week.

The new formats are:

• the "old" version, which doesn't specify a "base"

 "ft_value_list": {
        "2": 2,
        // ...
    },

• nested, with (or without) a range

 "ft_value_list": {
        "base": {
              // ...
        }
         "13->18": {
              // ...
        }
    },

A nested object with no base element is not valid and is treated dealt with an exit.