Support resource.$schema and no longer require tabular-data-resource

[peterdesmet]

Changelog: resource.$schema (new)

• Check $schema
  • If not provided, look for profile (see also Support resource.type #221)
  • If no profile, assume v1
  • If v1: investigate what effect is
  • If v2: interpret new properties
  • If v2+: warn that it may not understand all properties
• Update implementation
• Update documentation and mentions of profiles
• Update tests
• Update examples

In addition:

• No longer require tabular-data-resource (removed in this PR):
  • read_resource()
  • write_package()
• Update documentation https://docs.ropensci.org/frictionless/reference/read_resource.html#profile
• Update tests
• Update examples

Currently, data resources without the profile tabular-data-resource fail:

Error: Resource `validation_report` must have property `profile` with value `tabular-data-resource`.

I notice that resources without such a profile are quite common in the wild. These generic data-resource can use Table Schema in schema, but are not required to do so. Maybe read_resource() should be a bit more lenient and assess in a different way if a resource can be read. I think it already thus by e.g. requiring schema to contain fields.