Linear operator

[SebastianSemper]

We use the linear operator for all sorts of things now.

Especially to calculate largest eigen and singular values as well as the corresponding vector.

This pull request fixes a long and lasting series of complications with the power iteration.

[ChristophWWagner]

I have some questions and comments about the changes implemented that I would require to be discussed or resolved before proceeding:

• Why largestSingularVal and not go the full way to consistent naming largestSingularValue. I find the latter much more intuitive for a user as it is more consistent with the usual naming throughout the package and what the user would expect.
• The same for largestSingularVecs (should be largestSingularVectors), largestEigenVal (should be largestEigenValue) and largestEigenVec (should be largestEigenVector)
• Why is largestSingularVecs of type tuple? I would expect it to be a 2d ndarray, which would also allow slicing and would be consistent with the rest of the package (uniform interfaces, largestEigenVec is a ndarray)
• There should be FutureWarnings for largestSV and largestEV hinting the user to the API modifications, not hard API breaks.

[SebastianSemper]

I added an explanation of the largestSingularVector property to its doc string. right now it is the closest thing to numpy and scipy, since they both return a tuple with the same ordering of (u,v).