Internal configuration implementation, with overrides and env interpolation

[bschroeter]

Fixes #9.

Internal configuration implementation with default configuration as part of the installation.

Environment variables are passed in via Jinja templating through hpcpy's interpolation functionality.

This will allow us to modify cluster configuration via a YAML file, as opposed to editing source code directly.

I haven't touched the original config file in the root of the project, but we should move this out into a more appropriate place.

Tests pass, happy to walk you through the changes.

[bschroeter]

@abhaasgoyal, the tests are currently set up to really only work on NCI with access to specific data paths. We should update these tests to be in-memory (temp directory) or skippable on GH actions.

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[bschroeter]

Switched to in-memory paths for testing. @abhaasgoyal, why would the test be failing under different circumstances? push passes, but pull_request does not... In any case, we should probably not be running both if they are not producing the same results...