Define a repos.yaml like:

repos:
  experiments:
  - ../experiments
  - ../experiments/test
  # and systems, applications, and packages, see config/repos.yaml added in this PR for full example

in <benchpark_root>/config to customize where experiment definitions are pulled from.

Benchpark can pull config from one location, with the following priority (highest first):

• benchpark -C <dir>...
• If CWD where you call benchpark has benchpark-config directory
• If <benchpark_root>/config is a directory (this must exist if the first two don't)

More generally:

• Add a rudimentary YAML-based config system to benchpark
• Define a Repos section
  • Benchpark now uses this repos config to instantiate experiments and systems repos for benchpark itself, and packages/applications repos for Spack/Ramble
• Accommodates other possible sections
  • These sections support property-like access in python (but that assume the property exists, which they all should in the case of repos config)

TODOs

• several files are called repos.yaml or repo.yaml w/ different purposes, maybe I should call this new one something different
• schema validation
• Consolidate with benchpark bootstrapping config (currently gets consumed via a different mechanism, lives in a file in configure.yaml)