Add implicit workflow declaration to dsl2 template

[JoseEspinosa]

As discussed on this issue and merged on this PR of the rnaseq pipeline, DSL2 will include the implicit declaration of the workflow. This should enable that a nf-core pipeline can be imported as subworkflow other non-nf-core workflows as pointed out by @ewels in the module slack channel: "eg. run RNAseq analysis and then run whatever custom different expression analysis. or run rnaseq + atacseq and do x + y afterwards, etc etc".

Hence, the implicit declaration of the workflow needs to be added to the DSL2 template.