Allow an arbitrary number of model development runs

While CMEW must only handle a single reference simulation against which other runs are compared, in principle ESMValTool can process multiple test runs. Our NWP evaluation workflows will compare multiple test runs against a common reference, and it could be useful for CMEW to do the same.

See #142 for the variables that would need to be vectorised over the test runs.

Start by reading issue #142, which identifies the variables that would need to be vectorised over test runs. Trace how CMEW currently represents the single reference and test runs, then determine the workflow entry points involved. Done means CMEW can compare multiple test runs against one common reference without losing existing single-run behavior.

Written by the indexing model from the issue text.