Enable barcode and UMI length to be given, instead of a sequence of ?'s

[olliecheng]

At the moment, Flexiplex requires barcode and UMI sequences to be given as a sequence of question marks ("?"). This is a bit of a pain to work with, especially considering that the majority of use cases are likely going to have no specific requirements apart from length for the barcode or UMI.

This request is to track the feature addition of an alternative way to specify the barcode/UMI, by giving a length, similar to older versions of Flexiplex. For instance, -b ???????????????? would be equivalent to -b 16. Existing functionality would be retained in case more fine-grained configuration is required.