fix: a few purge_dups tweaks

[prototaxites]

Noticed a couple of little points for purge_dups that could be fixed:

• purgedups/purgedups outputs non-compressed BED file but purgedups/getseqs is able to read a compressed BED file, so this is compressed now.
• purgedups/getseqs hard-codes the -e option, which is not necessary for operation. This removes this in favour of allowing its specification in $args.
• Wrap all variables in ${} cause it's prettier :)

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[PeterKneale]

given the below: do you think it would make sense to have the -e parameter set as a default if no other configuration is supplied? would that be a safer transition for people already consuming this module?

[prototaxites]

Yes, that's probably a good idea

[prototaxites]

OK, fixed. I explicitly check if task.ext.args is a string, and if so we use it even if it's empty. Otherwise we fall back to -e, catching the case where it is unset and thus null.

[prototaxites]

@nf-core-bot fix linting