branch out RNG when measuring

• addresses #9
• needed !133 (merged) to work

this MR simply adds + Clone to rng and removes the mut from the rng of the measure_inbreeding function.

running the same snippet of code from #9 yields the following two images with a schedule of 1 and 5 respectively

we see that all measurements on t where t = 0 \mod 5 are the same in both images