add access to determinist metrics to `evalp`

This is an attempt to add access to determinist metrics to probabilist
evaluation (i.e. via the choice of a reducer on ensemble members).

The idea is to instantiate one determinist::Evaluator per site (in order
to reduce the dimension to 2D) as part of probabilist::Evaluator. This
way, the determinist evaluator handles all the memoisation/dependency
management, while the probabilist evaluator only needs to get the metrics
via a call to the getter methods of the determinist evaluator (e.g.
`get_RMSE`).

Unfortunately, while this commit compiles/links, it fails to run because
the views to q_obs/q_prd given to each determinist evaluator lose the
reference to the data as soon as it leaves the constructor method. So
there is likely a move/forward required somewhere, or the use of
a constructor method expecting universal references, but for now I could
not find a working solution along those lines. So I park this here for
now.