Added subworkflow mechanism and made miscellaneous changes

[ataseren]

I added subworkflow mechanism in DAG's and made some changes to improve the app overall.

Before merging it, extensive testing for all YAML's and features are advised.

I tested default YAML's, GoTTY and documentation features. I will continue to test but I suggest following a checklist that we decide on later.

[ErdemOzgen]

Thanks @ataseren , I will examined ASAP.