🐎 split agg metrics into individual tools

[dmiller15]

Description

Splits MultipleMetrics step into individual metrics collection steps. Increases number of parallel instances. Adds PR template. Updates documentation

Relates to https://github.com/d3b-center/bixu-tracker/issues/591 and https://github.com/d3b-center/bixu-tracker/issues/627

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] Documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• [x] Passes cwltool and rabix cli validation
• [x] WGS cavatica test: https://cavatica.sbgenomics.com/u/kfdrc-harmonization/kf-reference-pipeline/tasks/5c16a998-db9e-41d7-9d58-55006cdffa0d/
• [x] WXS cavatica test: https://cavatica.sbgenomics.com/u/kfdrc-harmonization/kf-reference-pipeline/tasks/ff2cc82d-2a52-4f57-b0b9-1227b98d78fd/

Test Configuration:

• Environment: Cavatica, Rabix 1.0.5, cwltool 3.0.20200324120055
• Test files: See cavatica tests

Checklist:

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] I have checked my code and corrected any misspellings

[dmiller15]

I'll take care of that tool naming problem in the main PR as it's unrelated to the contents of this PR.