Switching annotation mapping tables to long format

The goal is to achieve: 
```
cell_uid1, cell_id1, 20Q2|21Q2, DepMap
cell_uid2, cell_id2, 20Q2, DepMap
new_cell, cell_id3, 21Q2, DepMap
cell_uid3, cell_id4, 20210623, gCSI
cell_uid1, cell_id5, 20210623, gCSI
```