Suggestion: allow choosing measurement basis

[GiggleLiu]

Selecting the basis of a measurement plays a key role in measurement based quantum computation, which is an important branch of quantum computation (https://arxiv.org/abs/quant-ph/0301052) . Maybe recently I will have chance to learn and implement this feature. Following are two of my concerns,

• Although measurement basis problem can be circumvented through rotating bases, this approach is neither efficient nor compact.
• IBM devices do not support this kind of measurements for now, but I believe they will be needed in the future.

I want to know your opinions on putting this feature into ProjectQ. @damiansteiger @thomashaener

[thomashaener]

I think it should be straight-forward to implement, e.g., MeasureX as an additional gate and register a default decomposition rule which does the basis change. I don't see much benefit in adding this now since it would always be decomposed anyway but if it makes things easier for you / others, I'd be happy to accept a PR adding this.