Rethink `metrics` table

[nikosbosse]

In the old version there was a table called metrics with an explanation of all the available metrics. This method needs to be rethought in light of the new changes.

One idea would be to have a print method for the data that would print out additional information on the default metrics.

Related: #365

[nikosbosse]

My suggestion would be to remove the metrics table. The default scoring rules for every forecast type are documented in the rules_*() function documentation which links to all individual functions. In addition, users should be able to find more information in the Vignette.
I therefore suggest closing this issue and deleting the metrics object in #536. Please reopen if you disagree.