Poisson, Gaussian, Complex Gaussian with tests

[Pwhsky]

Builds on mg/noises/background to implement torch testing with the backend test class.

[mirjagranfors]

I have written some comments on the Gaussian noise class. Some of the comments also apply to the other classes. Once you have had a chance to address them, I can go through the remaining classes in more detail. Let me know if anything is unclear.

[mirjagranfors]

I think that we usually have two empty lines between different classes