Add human data from encode + bug fixes

[kelly-sovacool]

Changes

This PR includes a number of loosely-related fixes:

• add test human data from encode
• allow shared inputs across samples in deeptools subworkflow
• allow additional columns in samplesheet beyond minimal header
• change peak widths histogram type from overlay to stack
• add workflow entry point to download SRA (not yet integrated with rest of workflow)

Issues

towards #165, #169 (does not resolve)

PR Checklist

(Strikethrough any points that are not applicable.)

• This comment contains a description of changes with justifications, with any relevant issues linked.
• [ ] Write unit tests for any new features, bug fixes, or other code changes. testing framework not yet implemented
• [ ] Update docs if there are any API changes. on hold until before public release
• [ ] If a new nextflow process is implemented, define the process container and stub.
• Update CHANGELOG.md with a short description of any user-facing changes and reference the PR number. Guidelines: https://keepachangelog.com/en/1.1.0/