Organize each variant caller into its own subworkflow

[rnaidu]

Description of feature

Re-organizing nucleovar.nf to make sure each Variant Caller (VarDict,MuTect,MuTect2) is called in a separate subworkflow. This is in order to compartmentalize the code and keep nucleovar.nf concise.

Steps in each variant caller subworkflow:

• Variant Caller module
• Variant Caller Filtering
• BCFTools Suite (gzip,norm,sort,concat)

Final output of each subworkflow is a concatenated VCF file that will be annotated against each variant caller.

[rnaidu]

included in most recent PR: #17 (comment)