Feature/tamg

[pohm01]

Add a matrix-free h-multigrid.

Currently only handles piecewise constant interpolation between grid levels.
Chebyshev as a smoother and coarse gird solver.

[timofeymukha]

Very cool stuff, Peter!

Some quick comments

• Don't use print, we stick to write
• You may want to look at the suggestions of the linter under the develop/Flint/List chagend files check. At least sticking to col80 lines would be very nice.
• This is CPU-only, right? Do we have a check in the code?
• We should add the tamg to the documentation.
• Related to the above, when would one use this one as opposed to the hsmg. Is there an example where we could perhaps use it?

[njansson]

Some initial fixes for (C) year (the year should be when the file is added)