This fixes a couple issues that were causing qtlreaper to not work on the DOL (Diversity Outbred Lung) genotypes.
For some reason the new qtlreaper was sorting chromosome alphabetically, so I addressed this in a previous commit by sorting them using an algorithm that sorted numbers followed by alphabetical letters. But this caused issues with the introduction of "M" as a "chromosome" - it's last in the genotype file, but wasn't being sorted last by the sort algorithm. So as a kind of awkward fix, I made the sort algorithm treat "M" as "Z" (making it always be last)
Doing interval mapping would cause and error if there was only one marker for a chromosome (since when drawing the figure it links individual markers with one another within each chromosome). I changed it to ignore edge cases where there's only a single marker on a chromosome.
This fixes a couple issues that were causing qtlreaper to not work on the DOL (Diversity Outbred Lung) genotypes.