Consider allowing an arbitrary number of model test runs

[alistairsellar]

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.