Add mobile element calling into the pipeline

[peterpru]

Description of feature

From talking to clinicians: As a clinician, I need the pipeline to be able pinpoint mobile element insertions in our genomic data for detecting disease-causing variants.

Link to nf-core issue: nf-core#329

Current suggested solution: Add a retroseq subworkflow as implemented in MIP.

Actions:

• Add retroseq discover as local module
• Add retroseq call as local module
• Add call subworkflow
• Add annotation subworkflow

PR for nf-core:
nf-core#440
nf-core#483

[jemten]

done in dev branch and awaiting new release