Protein substitution model, using the dcmutt implementation of Kosiol
and Goldman 2005 and free equilibrium frequencies. The thetaX
are frequencies parameters, where X is 1 to 19. Parameter theta1
is the proportion of A, theta2 is the proportion of R over
(1-A), theta3 the proportion of N over (1-A-R), etc. See the
Bio++ description.
Build a protein substitution model from a file in PAML format, and use
free equilibrium frequencies. 'name' will be used as a parameter
namespace, including for frequencies.