Allow converter deferral and move Reference to a converter

[braingram]

Following the ASDF tag-up discussion here is another option for moving Reference to a converter based on the suggestions from @eslavich

asdf.reference.Reference serializes to an untagged dictionary.

asdf/asdf/reference.py

Lines 45 to 46 in b0c9a50

	class Reference(_types._AsdfType):
	yaml_tag = "tag:yaml.org,2002:map"

asdf/asdf/reference.py

Lines 108 to 111 in b0c9a50

	@classmethod
	def to_tree(cls, data, ctx):
	uri = generic_io.relative_uri(ctx.uri, data._uri) if ctx.uri is not None else data._uri
	return {"$ref": uri}

This type of conversion is unsupported in the new extension API (using Converters) in asdf main. To allow Reference to be moved to a Converter this PR introduces the ability to define converters that defer conversion to other converters (or in the case of Reference to un-tagged serialization to a yaml map).

This PR changes the public API of Converter.select_tag (in a backwards compatible way) allowing it to return None when asdf should convert the object being serialized with a different converter. When select_tag returns None, the deferring Converter.to_yaml_tree is called on the object being serialized to convert the object instance from one type (the one that triggered deferral) to another type (which can either then defer to another converter or return a valid tag and serialize the object).

In the case of Reference the new ReferenceConverter.select_tag returns None and ReferenceConverter.to_yaml_tree returns a 'vanilla' dictionary which will be converted to an untagged yaml map.

This converter deferral can be used to support subclasses that can be serialized as the superclass (which was done automatically with the old extension API) and docs were updated to describe options for how subclasses can be supported. The changes in this PR provide a migration path for user code that previously relied on ndarray subclass conversion as described in #1537

[braingram]

py38-devdeps failures are expected and addressed in: #1556