feat(medcat): CU-869cw9zmj Improve inference speed

[mart-r]

This PR improves inference speed somewhat (around 10%) by:

• Using a more efficient unitvec calculation
  • This gave a nice boost to speed (around 20% on this segment)
  • But it was doing the change in place so that polluted other bits which broke a bunch of stuff
  • However, this speedup came from a very small section of the overall time
  • So the overall speedup of this would have been closer to 1%
• Reusing smaller context vectors mutiple times
  • We've cont multiple context windows (small, medium, large, extra larget)
  • So far we've been recalculating the vectors for all of these for every context window
  • However, the smaller ones will always need to be a subset of the bigger ones
  • So this change will take advantage of that