@abc.abstractproperty<EOL><INDENT>def branchScale(self):<DEDENT>
pass<EOL>
Factor to scale branch lengths to substitutions per site.

        When we use a branch length of `t = 1`, how many substitutions
        per site do we expect averaged over all sites? This method
        returns that number. So to scale branch lengths to units of
        substitution per site, multiply by this number.