Feature/254 sp15 item12 create nifti dataloader for loading medical images

Closes #254 (closed)

Changelog

• Unit testing added (including synthetic data generation).
• Lenght property of the dataset adjusts depending on the available information.
• Compatible with Dataloaders. Returns a dict of dict being the first the data source data, target, demographics and the other the modalities within each e.g. T1, T2, label, AGE, CENTER_NAME.
• Multi-format support for demographics file (tabular_file): CSV, TSV, Excel.

Todo for #290 (closed)

• Add necessary attributes, properties, and methods for GUI.
• Adjust the model gathered by @sayed so data can be extracted in a compatible manner at each training_step.
• Finishing refactoring by @mvesin

Extensions:

• Create class called BIDSBase where BIDS folder structure and reference verification operations are provided
• BIDSDataset inherits BIDSBase
• Create BIDSController that inherits BIDSBase. This class is used for dataset deployment actions.