Update the M7 model, M8b_Test, and other models.
Sample frequencies less frequently.
Fix logging of a and b.
Change omega to follow a uniform distribution under the M0 model.
Update M1a, M2a, and M2a_Test models.
Rename categories to start with 1 instead of 0.
Change M2a_Test and M8b_Test to set posP to 0 when no positive selection.