Review complex number representation

[eslavich]

Currently our schema for complex numbers specifies a complicated string-based representation:

https://github.com/asdf-format/asdf-standard/blob/master/schemas/stsci.edu/asdf/core/complex-1.0.0.yaml

Is there an advantage to storing these as a string? If not, we should consider switching to a list of two numbers or two numeric properties, since parsing the string is difficult.