Add Integrator2D base class

[Mateasek]

This is the base class for Integrator2D proposed in #472

I mostly copy-pasted the Integrator1D by @vsnever

[vsnever]

Hi @Mateasek,

Thanks for this PR. The Integrator2D base class takes y_lower and y_upper as doubles and not as Function1D objects. In general, the boundaries for y should be a function of x. Does your interface cover all the integration cases in #471?

[Mateasek]

Hi @vsnever,

In general, the boundaries for y should be a function of x.

This is a good point, I will change it.

Does your interface cover all the integration cases in #471?

I hope so. The model I'm working on (at least for now) needs to integrate only over 2 dimensions. I simplify the radiated spectrum, otherwise it would need 3D integrator which for now seems unnecessary and too computationally demanding. If 2D is not enough, I can always add 3D integrators and we will have 2D for future usage in the worst case.

[vsnever]

I think it's perfect now! Thanks.

[Mateasek]

Thanks for review @vsnever