Conditional cnv

[kathryncrouch]

Ensure downstream steps in the DNAseq graph don't fail if CNV steps have not been run in Nextflow:

• Modified the WorkflowStep class that writes the loader config. If it cannot find CNV or ploidy input files, it will not write these rows into the config. Thus, the loader will not try to load them. No changes to loader. This will still fail if the input file for indels cannot be found.
• The WorkflowStep class that copies bigwig files to webservices uses nullglob in the cp command. If one or more bigwig files are found, they will be copied. If no bigwig files are found, the pattern expands to a null variable and the cp command does not fail.