Multimodal Source Decomposition Notebook and Toy Data

[TCodina]

• Adds example notebook on multimodal source decomposition on synthetic fNIRS-EEG data
• Creates the sim/synthetic_toy_datasets subfolder and the synthetic_fnirs_eeg.py script inside, which simulates bimodal data as a toy model for concurrent EEG–fNIRS data.
• Minor changes in the cca, and tcca implementations, default values, and documentation.