[feature] Keep signal-file metadata in separate dataframe

[WilliamCappelletti]

Is your feature request related to a problem? Please describe.

Much information is unnecessarily duplicated in the segments dataframe (e.g. date and sampling rate). We could move all that to a separate dataframe indexed by file name.
In there we could also include

• Available channels
• Path to original file

Describe the solution you'd like

A separate file would carry all that information.

Problems

We would have to provide another dataframe to EEGDataset, unless the segments df point to the metadata instead of the actual file (this requires reading two files at each call).