Implementation of Logger

[jarlebring]

This is an attempt to incorporate a logging functionality #170.

As a user: You can use the new logger kwarg for the NEP-solver:

• If logger is an Int, it will behave as displaylevel before, under the hood a PrintLogger is instantiated.
• If logger <: Logger one can obtain more detailed control of what is printed and one can store iteration info in the logger object.

As a NEP-method developer:

• push_info!(logger,"text") can be used to dump info to the logger. If the logger is a PrintLogger "text" will just be printed. Note that you can add variables by using the dollar sign: "x=$x"

• push_iteration_info(logger,iter,kwargs...) can be used to push information about iteration iter. kwargs include things such as err, v, etc

• method_newton

• iar tiar

• ErrorLogger storing information

• rfi

• nlar

• infbilanczos

• contour integrals

• nleigs

• sgiter

• mslp

• jd

• ilan

• broyden (w inner solves)

• block newton

• unit tests

• comments + documentation

Done with: a level parameter: At the moment I am not sure how to handle iar and other methods which compute many eigenvalues. Just printing all the eigval approximations by default would be give too many printouts, but we want this information when we want to study the method (e.g convergence diagrams).

[jarlebring]

Okay. Starting to feel ready with this PR.

[eringh]

I agree. It starts to be ready to merge.

[jarlebring]

I like it. Time to merge.