Reject nonexistent fields in `pset create`

[schoen]

Currently, hal-simplicity simplicity pset create will accept spurious field names in an input. You can just make up any field name in the JSON, and it won't raise an error. This appears to be wrong.

Interestingly, the problem seems slightly different for outputs; some spurious values may cause the entire PSET to be null (without a specific error message).

Ideally, both inputs and output arguments should be validated to catch errors early.