Gaussian backend claims it can do operations that are not implemented

[cgogolin]

Specifically this concerns:

• 'DisplacedSqueezed'
• 'TwoModeSqueezing'

At the same time, operations provided in ops.py are commented out, namely:

• 'XDisplacement'
• 'ZDisplacement'

I would fix it myself, but due to the move, I don't have the time this week, so I am at least taking a note.

I discovered these while working on general unit tests for the installed plugins here.

[josh146]

Ah, those operations were a bit forgotten. I was in two minds when coding that - for simplicity, I only included a single Displacement operation in OpenQML, since displacement in x and displacement in p are only special cases.