Rename some things; make proper R func for sim_meiosis and sim_crossovers
Working towards obligate_chiasma option

 - added argument; now need to implement its use in sim_crossovers
Add tests of calc_Lstar (copied from R/xoi)
Fix typo in docs; update .Rd files
Update ToDo list
Implement obligate_chiasma

 - had to revise expected test results, due to reordering of some of the
   code
Fix bug in sim_crossovers for m=0 obl_chi=FALSE case
Fix another bug in sim_meiosis. _Now_ obl_chi is working