expand_mutation_string: Combine the columns describing a mutation into a single...
In abelew/Rerrrt: Examine RT error rates through sequencing.

Given a table with columns including the position, mutation type, reference nt at that position, and product nt at that position; create a single column from it, standardize it, and make some categories describing each mutation. These columns currently include: 'mt': X_Y telling that this is a mutation from X to Y, 'transition_transversion': this is a transition or transversion mismatch, 'strong_weak': this mismatch went from strong->weak, weak->strong, weak->weak, or strong->strong. undef is used in the case of indels.