metrics: calculator: support probabilistic forecasts

[dplarson]

Related to #266: metrics.calculator needs to be revised to support probabilistic forecasts. (Currently only deterministic forecasts are supported, but the framework is there to extend to probabilistic forecasts.)

[awig]

TODO: for calculator.py

• [x] create probabilistic forecast related pytest fixtures in conftest and test_calculator
• [x] add support for ProbabilisticForecast in calculate_metrics
  • [x] TEST - test_calculate_metrics_with_probabilistic
• [x] add calculate_probabilistic_metrics
  • [x] TEST - test_calculate_probabilistic_metrics_no_metrics
  • [x] TEST - test_calculate_probabilistic_metrics_no_reference
  • [x] TEST - test_calculate_probabilistic_metrics_no_reference_data
  • [x] TEST - test_calculate_probabilistic_metrics_missing_data
  • [x] TEST - test_calculate_probabilistic_metrics_no_observation
  • [x] TEST - test_calculate_probabilistic_metrics
• [x] add _apply_probablistic_metric_func
  • [x] TEST - test_apply_probabilistic_metric_func
  • [x] TEST - test_apply_probabilistic_bad_metric_func
• [x] add helper transform function
  • [x] test