Cleanup parsing of file pairs

[dleehr]

We raised some design questions about the sample/file picker in #115

Remaining issues:

• Support parsing of sample names from proposed convention (split by _ but what component is what? Needs clarification.
• Removing a sample from the middle of the list causes all below samples to reshuffle and lose any custom names. Instead, only allow whole-sample deletions.
• Add a Remove all button to the list of samples.