Proper truncated density for guided distal length

Verified that this integrates to 1 via ocamlgsl:

    (* utop *)
    #require "gsl"
    ;;

    let lower = 0.0
    and upper = 1.0
    and mu = 0.5
    and sigma = 0.6
    ;;

    let epsabs = 1e-4
    and epsrel = 1e-4
    ;;

    (* Without rescaling *)
    let f x = Gsl.Randist.gaussian_pdf (x -. mu) sigma
    ;;

    Gsl.Integration.qng f ~a:lower ~b:upper ~epsabs ~epsrel
    ;;
    (* - : float * float * int = (1., 1.11022302462515654e-14, 21) *)

    let f' x =
      let num = Gsl.Randist.gaussian_pdf (x -. mu) sigma
      and denom = (Gsl.Cdf.gaussian_P (upper -. mu) sigma) -.
        (Gsl.Cdf.gaussian_P (lower -. mu) sigma)
      in
      num /. denom
      ;;

    Gsl.Integration.qng f' ~a:lower ~b:upper ~epsabs ~epsrel
    ;;
    (* - : float * float * int = (1., 1.11022302462515654e-14, 21) *)
lcfit: fall back on standard guided proposal

cc: @bcclaywell
A few more comments
Merge branch 'master' into 75-uniform-proposals
Factor out distal, pendant proposals