Split `metrics()` into function for listing available metrics and calculating metrics

[dgkf]

Currently, metrics() is a bit overloaded with functionality. It provides an interface to both

• view available metrics using metrics() which returns a list<pkg_data_info> (list of metric metadata)
• calculate metrics for a package, which returns a list<class_atomic> (list of metric results)

I think this is too unclear and we should probably split them up.

Proposed split

• available_metrics as a generic over parameter x, such a that

  • x is of class class_character: returns an available.packages()-style matrix, currently implemented in val.criterion:::available_metrics
  • x is of class class_pkg: returns the metrics that can be derived given the package's specific resources
  • x is a class_resource: returns the metrics available for a specific resource type

• derive_metrics which calculates the metrics themselves. Only needs to be implemented for class_pkg and maybe list<class_pkg>... not even sure it needs to be generic.