Workflow module for Niffler framework

Niffler provides various modules to handle different DICOM related tasks. Some of the modules are to be executed sequentially by the user to get the desired output. This project aims to reduce human intervention by building flexible and scalable pipelines using Nextflow. Nextflow is a domain-specific language for workflow orchestration. It enables scalable and reproducible workflows using software containers. Nextflow allows us to add new modules without having to hard-code the pipeline.