split hmm.h into separate files
est_rec_frac for riself and risib: call basic function first
est_rec_frac for f2pk: denom == n_ind
est_rec_frac for X chr in f2pk: call the basic func
est_rec_frac for RISIB on X chr; need to test
est_rec_frac: use fact that denom = n_ind; just need to ensure result >= 0.0