Ensure complex number support

[sneakers-the-rat]

From: LiberTEM/LiberTEM-schema#7 (comment)

Need to

• Add complex number tests to all the dtype tests
• Split up the all-interface tests to make them combinatoric with a product of test cases, so we can test the all-interface behavior across all dtypes and shape cases.
• Ensure that one can round-trip complex numbers to/from json

This will probably be the point where we need to start developing dtype adapters for interfaces that allow them to coerce/serialize different dtypes that they do/don't support in different ways.