Add ability to save paired-FASTQ files

There is existing functionality for loading a pair of FASTQ files, but not for saving such files. I am in need of a tool to convert some BAM files to paired-FASTQs, so I added this capability.

I'm not an expert on the convention of having read names all end with `/1` in one file and `/2` in the other; for my purposes I want to always write them out that way and have the option of doing some strict sanity checking (all reads have a mate on the way in and out), so I added a few optional levels of verification to the loading and saving steps.

ah yea, woops, changed the other instance of this pattern, on the save path.

this load path is only called from tests atm; let me look into changing it anyway for consistency