Add guessing lineages and previewing lineages (first iteration)

lineage/lib/__init__.py
-----------------------

Lineage class:
- Move UUID and lineage loading from `load_cells` function into
  dedicated `load_uuids` and `load_lineages` functions.
- Rename `get_centre` to `get_box_centre` and add new `get_centre`
  function as a replacement. `get_centre` now uses the middle point of
  the median element of the mesh, so should be more accurate than the
  average of all the outline coordinates. `get_box_centre` is identical
  to the previous method.
- Add function `get_overlap`. Currently unused, but constructs circles
  over two cells in subsequent frames and calculates the area of
  intersection between them, and modifies this by a factor that reduces
  the area depending on how different the angle of cell axis is.
- Add function `get_distance`. This function calculates an average
  distance between two cells in subsequent frames, between both poles
  and the cell centre. New functions `_get_poles` and `get_pole_distances`
  are called by `get_distance`.
- Add function `search_daughters`. This function attempts to assign
  daughters to a given mother cell. Based on the knowledge that daughter
  cell poles will be aligned to the cell centre and one of the parent
  cell poles.
- Vastly fill out `guess_lineages` using `get_distance`, and
  `search_daughters`.
-  Add `write_lineages` to avoid repetition of code.

LineageMaker class:
- Colour potential daughter cells in blue and cyan instead of both green
  for additional clarity.
- Attempt to avoid 'greying' and inversion of subplots when cells are
  close to the frame boundaries (needs more work).
- Add preview function (work in progress, seems to forget to add the
  initial starting cell). Displays every frame of an assigned lineage,
  and allows accepting (ENTER) or rejecting (CTRL) it.
- Added the grand-mother cell outline in `next_frame` when starting from
  a child cell.

lineage/lineage-bin
-------------------
- Added both guessing and interactive tracking to execution.
Docs update