C10_fixed — Mixture model

Empirically derived mixture model where different components have different equilibrium frequencies to account for site-specific biochemical constraints. It partitions protein alignment sites into 10 distinct profile classes, each with a unique amino acid distribution, to better approximate the varying selective pressures across a sequence.
This site-heterogeneous approach effectively mitigates long-branch attraction and systematic biases that often compromise simpler, single-matrix models like LG or WAG.
This version fixes the relative frequencies of the 10 components to the values from the paper.