Configuration: Samples Sheet validation

[AlexTate]

A new class, SamplesSheet, has been added to configuration.py. It acts as a basic API to the Configuration class and validates the contents of the user's Samples Sheet. If the following requirements are not met, it is treated as a hard error and pipeline execution halts at startup:

• Only one group is designated the control group for tiny-deseq.r compatibility (otherwise, the error message suggests that the user remove all control designations so that every possible comparison is made instead)
• Each group-replicate combination cannot be listed more than once
• Fastq files cannot be listed more than once
• Fastq files exist (relative paths are resolved prior to checking)
• Fastq files have a .fastq or .gz extension

If the following requirements are not met, a warning is printed and pipeline execution continues:

• The experiment design has at least one degree of freedom for compatibility with DESeq2. Otherwise, the step is automatically skipped.

Closes #242

[AlexTate]

This update should also be included in the upcoming release; please merge prior to PR #241

[taimontgomery]

Tested successfully with ram1 data.