Implement our own Factor class to mimic R's behavior.

[LTLA]

• tests
• str method
• fixes: Don't attach row names during subsetting #50, Generalize row/column index subsetting to any Sequence[int] #26

[LTLA]

Needs #49 to be merged first, because I started work on the wrong branch 🤭

[LTLA]

Also looks fine in the BiocFrame show method:

>>> biocframe.BiocFrame({ "Aaron": biocframe.Factor([1,2,3,4], levels=["A", "B", "C", "D", "E"]) } )
 BiocFrame with 4
      rows &
  {self.dims[1]}
  column{'s' if
self.dims[1] != 1
     else ''}
┏━━━━━━━━━━━━━━━━┓
┃ Aaron <Factor> ┃
┡━━━━━━━━━━━━━━━━┩
│ B              │
│ C              │
│ E              │
└────────────────┘

[jkanche]

also fixes: #50

[jkanche]

and fixes: #26