Add the DataLoadingPlan functionality to MedicalFolder*

Closes #319 (closed). Should not be merged before !131 (merged) .

The class diagram looks like this. I have highlighted in green the functions that have been modified, and in red a new special function that was needed to be able to use the DataLoadingPlan inside a MedicalFolder

The main changes are:

• MedicalFolderBase now inherits from DataLoadingPlanMixin, thus giving the DataLoadingPlan functionality to both MedicalFolderDataset and MedicalFolderController.
• The DataPipeline DataLoadingBlock implementing the mapping of Modalities to Folders is simply an instance of MapperDP MapperBlock. Its DataLoadingBlocks __type_id should be modalities_to_folders. Its map member has the following semantics: map = {'modality': ['folder1', 'folder2', ...]}. For example

map = {'T1': ['T1-Philips', 'T1-Siemens'],
       'T2': ['lowdose', 'myscans'],
       'label': ['label']
       }

An example of the full usage can be found in the tests, and will be made even more clear in the future when we implement CLI and GUI.

• Before, the MedicalFolderController scanned all subfolders of all subjects to identify all available modalities. Now the modalities obtained in this way are considered "candidate" modalities (for instance, to be presented to the GUI). The candidate modalities are the same as the actual modalities only when a DataLoadingPlan is not active, otherwise the actual modalities are obtained from the DataLoadingPlan
• We no longer cache the subjects_has_all_modalities property, because if a DataLoadingPlan is set after this property was computed the first time, it would become invalid
• The _subject_modality_folder has the important role of providing a link between the purely logical map between {'modality': ['folder names', ...]} and the actual situation on the file system. When we interrogate the DataLoadingPlan for the folder names corresponding to a given modelity, we obtain a list of possibilities. But which one is right depends on which one actually exists among the subject's subfolders. The _subject_modality_folder is responsible for finding this.