Rename "sr" preset and lock it out of paired-end modes so it doesn't look like the right choice for short reads

[adamnovak]

Right now lr-giraffe has an sr preset which is for single-ended Illumina reads with chaining.

If you use it for paired-end Illumina reads, it can't do chaining, but it also still changes a bunch of parameters and givesd bad results with the non-chaining codepath.

We should rename it, and probably lock it out of paired-end modes.