Character-level decoding (ERP and c-VEP)

[PierreGtch]

Character-level (or trial-level) decoding of ERP and c-VEP (see #366) datasets would allow to benchmark the state-of-the-art algorithms that directly exploit the trial structure instead of doing a binary target/non-target classification.

For such a trial-level decoding to be possible in MOABB, we would need to:

• [ ] #186 ;
• [ ] pass the stimulation sequence of each character to the decoding algorithms.