Fix bug in simulation for Stahl model

 - for simulation of no. chiasmata from process with no interference,
   had the wrong mean; was L/100*p but should be L/50*p

 - also added a couple of R:: for calls to runif()
Avoid repeated calls to RNGScope

 - really need it in sim_meiosis
 - not needed in sim_crossovers unless called from R
Oops! some of those R:: should've been Rcpp::
Back to avoiding repeated calls to RNGScope

 - also ensure call to RNGScope within sim_meiosis

 - doesn't pass tests, but that's because of the bug fix for Stahl model
   (don't commit until you've run the tests!)
Fix tests of sim_meiosis

  - bug fix re stahl model results in different simulation results in
    tests/testthat/test-sim_meiosis.R