Implement numba algorithm to speed-up genetic_value.py #190

In reviewing tstrait/genetic_value.py @jeromekelleher noted at #189 (comment) that: "We should leave a comment here to the effect that this implementation is slow and we'll probably need to replace with a numba algorithm. The mechanisms added in tskit 1.0 should make this straightforward though. Let's leave for a follow-up rather than trying to do here."