Handle case where target coverage is above estimated coverage of input

[dfornika]

In the case where the target coverage is above the actual estimated coverage of the input file, we should not produce a fastq output.

Should an entry still be made in the summary spreadsheet? Maybe something like this, with NA for any target coverage that couldn't be achieved?

sample_id  total_reads  total_bases  q30_rate  target_coverage
sample-01  42158        10000353     0.881622  2
sample-01  105510       25000351     0.883446  5
sample-01  210932       50000154     0.883396  10
sample-01  421748       100000287    0.882951  20
sample-01  632628       150000289    0.88305   30
sample-01  843538       200000061    0.883072  40
sample-01  NA           NA           NA        200
sample-01  1172548      278027194    0.882882  original