v0.49.0

Added

• The substitution rate can vary among gene families:
  substitution=PerSite(1.0).varying_among('families', LogNormal(0.0, 0.8)) draws one multiplier per
  family before any site evolves, and the family keeps that value for its whole life. It multiplies
  the lineage clock rather than replacing it, so a branch of a gene tree is
  substitution × Δt × lineage clock × family multiplier substitutions per site. The multipliers are
  written in the sequence run's own family_multipliers.tsv, with a substitution column, and are
  .family_multipliers in Python; runs with parallel= or stream_to= write the file too. A
  nucleotide sequence run refuses the draw, because it evolves blocks rather than gene families. (#444)