Introduce experiment evaluation

This MR covers the following points:

• add a MetricFetcher, closes #139 (closed)
• introduce experiment evaluation, closes #135 (closed)
• add a Python slope checker component, closes #136 (closed)
• Resolves #168 (closed)

TODOS:

• Make hard coded urls configurable
• Clean up
• Add instructions for execution (README)
• Handle different time zones between Theodolite and Prometheus