Write an initial set of metrics

[ma595]

Write an initial set of metrics (and tolerances) that can be used to physically validate the ML projection.

[ma595]

@GuillaumeGACHON provided an overview of these in the meeting today. @vopikamm also mentioned that he's written his own version of these metrics. Code is available here.

We discussed in the first instance how the metrics capture general physical trends in all ocean models, but the tolerances themselves may be tuned to individual ocean models.