Update complex number schema and add tests

[drdavella]

This PR addresses several outstanding problems with the schema for complex numbers. It adds much more thorough tests of the schema and ensures that improperly formatted complex numbers are detected by a schema validation error (which was not previously the case).

In a way, this is a fairly substantial change since it imposes much more stringent requirements on the format of complex numbers. However, since these requirements were implicit to begin with and should have been enforced in the first place, it seems unlikely that anyone is using files that are in conflict with the new rules.

This is closely related to asdf-format/asdf-standard#171 and integrates the updates to the schema that were introduced there.

[stsci-bot]

Hi there @drdavella 👋 - thanks for the pull request! I'm just a friendly 🤖 that checks for issues related to the changelog. I help make sure that this pull request is milestoned and labeled correctly. This is mainly intended for the maintainers, so if you are not a maintainer you can ignore this, and a maintainer will let you know if any action is required on your part 😃.

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[coveralls]

Coverage decreased (-0.003%) to 93.798% when pulling 2fee7ff on drdavella:add-complex-tests into 37d57aa on spacetelescope:master.

[drdavella]

Added a change log entry since this change has the potential (however minimal) to affect the behavior of asdf when reading files with ill-formed complex numbers.