Improve ObjectMapper and fix related issues

[rly]

Motivation

• Fix Error with unnamed collections of links #453. When handling collections of links, ObjectMapper.convert_dt_name(...) now uses the name of the LinkSpec target type to generate the attribute name instead of throwing an error.
• Raise a warning if a GroupSpec has a collection of groups/datasets/links with quantity '+' and is built with no objects. See also Validator should check for quantity #197.
• Fix Containers in a subgroup can get built twice #342. No longer build containers that do not match the data type of the given spec.
• Remove extra code in ObjectMapper that caused some objects to be built multiple times.
• Allow writing of collections of empty Container and Data objects. These would previously not be written. Fix empty Containers not writing #224.
• Restrict NamespaceCatalog.is_sub_data_type(...) to accept only strings and refactor.
• Raise ContainerConfigurationError if a container class has no attribute mapped to a spec.
• Replace some ValueErrors / plain Exceptions with BuildErrors / ConstructErrors. See Plain base Exception instances should not be raised #340.
• Create alias MissingRequiredBuildWarning for MissingRequiredWarning. Use the new warning throughout.
• Enhance BuildManager.is_sub_data_type method to support checking whether a given AbstractContainer is a sub-data_type of the data type defined by a Spec.
• Remove impossible-to-reach code from GroupSpec.set_link(...)
• Remove impossible-to-reach code from ObjectMapper.get_builder_name(...)
• Make ObjectMapper warn for incorrect quantity and missing cases for required groups. Related to Validator should check for quantity #197
• Add hdmf.testing.assertBuilderEqual(...) which compares builder names

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[rly]

The changelog needs to be updated after this is reviewed.