Split up the entity table into smaller sections

[sappelhoff]

The entity table in the appendix is currently one large table for all data modalities .

It might make sense to split the table into several sub-tables (e.g., MRI, MEG, EEG).

1. This would make each table more readable
2. Users are typically interested in one modality at a time, thus having modality-separated entity tables would decrease noise while reading
3. Having entities split up by modalities would make it easier to insert links from each entity to an in-text description of the usage of that entity in the context of the specific modality

Note: This issue can be worked on independently from #289 ... solving both issues would be a great improvement.