Handle case where fastq_dir is a symlink

[nathanweeks]

Fixes issue #124 , checking if for a non-null/"file" scheme before attempting to resolve symlinks to avoid breaking cloud storage paths as supported in PR #122.

I manually tested (using --profile singularity) the case where a fastq_dir specified in the sample sheet is a symlink---not sure if it's worth adding an explicit test? Also, note that I'm only a "groovy novice", and suggestions containing a more robust or conventional implementation would be appreciated.

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