Slight progress on lineage assignment

Add `centre` attribute to all `Cell` objects which is set by the new
`Lineage._centre` function.

Add some basic progress on the `guess_lineage` function by calculating
the area of overlap between two circles.
Each circle is centred on a cell, with radius of the cell length.

Plan is to assign daughters first based on the distance between cell
centres, then by percentage overlap (percentage of the mother cell
circle).
Disable interacting tracking for dev purposes
Update docs