range usage

[snopoke]

The usage models assume that a field is consistent throughout the entire time frame but that's often not the case. In order to allow a usage field to change over time it should be possible to define what timerange a usage field is relevant for so that multiple fields can be combined to form the final field. For example:

Phase 1 SMS: Beginning to 2018-01-01
Phase 2 SMS: From 2018-02-01

usage:
  sms_phase1:
    model: 'derived_factor'
    dependant_field: 'users'
    active_range:
      lte: '20180101'
    factor: 120
  sms_phase2:
    model: 'derived_factor'
    dependant_field: 'users'
    active_range:
      gte: '20180201'
    factor: 240
  sms_total:
    model: 'derived_sum'
    dependant_fields:
      - sms_phase1
      - sms_phase2