This is the result of a discussion I had with @CallumAttryde There is a clear need to be able to check if an operation is a valid gate or pulse. We have a consensus on the truth table (how it should behave), but we do not yet agree on the implementation. Callum proposes an implementation that uses the python type system. Adriaan does not understand the proposed implementation yet and proposes to use the `@property` tag instead.  Adriaan proposes to add the boolean properties `valid_pulse` and `valid_gate` to the `Operation` class. This discussion was triggered by #22 and #6.