additional test in check_data_integrity.py

[gregcaporaso]

Should check that linked files have:

• one of the following names: mock-forward-read.fastq.[gz|zip], mock-reverse-read.fastq.[gz|zip], mock-index-read.fastq.[gz|zip]
• no duplicated names
• links exist for forward and index files (reverse is optional)

[nbokulich]

Also: automatically check validity of raw data, e.g., confirm that files are readable and (possibly) in valid fastq format (issue with the latter is that, depending on the software we use and the age of the dataset, header lines that are valid fastq may not be recognized by the software)