Throw a better error when `params` is missing from an operator

[astanziola]

Is your feature request related to a problem? Please describe. The current error that is returned to the user when params is potentially not clear:

SignatureError: The argument 'params' must be a keyword argument in . Example: def evaluate(x, *, params)

Describe the solution you'd like Make it clear to the user that params must be defined for operators, and give an example of how that must be done. At the moment, it relies on plums signature error handling

[astanziola]

Fixed in #139