Separate data providers voor training/test

[wowtor]

See #315

Goal is to enable use-cases:

• calculate LLRs on a separate data set (e.g. case LLRs)
• use a pre-trained model (no training data required)

Current situation:

data:
  provider:
    method: parse_features_from_csv_url
    url: ...
  
  splits:
    strategy: cross_validation_sources
    folds: 5

New situation:

• new property: data.training, data provider for training data
• only one of data.splits or data.training is allowed
• if data.training is available, use data.provider as test data
• if both data.training and data.splits are unavailable, use all data as test data

data:
  training:
    method: parse_features_from_csv_url
    url: ...

  provider:
    method: parse_features_from_csv_url
    url: ...