Class for smoothness indicators

[vladotomov]

@HennesHajduk , please make sure that your smoothness indicators test still give the same numbers. The ones in the README look fine.

This just moves all the related code in a class SmoothnessIndicator.

[HennesHajduk]

The FCTSolver class does not use the smoothness indicators (neither does the ClipScaleSolver class).
So the results for ./remhos -mt 4 are the same if you use smoothness indicators 0,1 and 2.
Please include them according to the currently unused routine ComputeFCTSolution.

I will then rerun the tests in our paper and see if I get the same results.
Thanks

[vladotomov]

@HennesHajduk I included the smoothness indicator calculations to the new FCT classes. Please check if it's correct.

[HennesHajduk]

I re-ran the following tests corresponding to our Computers & Fluids paper:
Fig. 2, d-f (Smoothness indicator 1 / formula (37))
Fig 3 c, f (Smoothness indicator 1 / formula (37))

The results are the identical.

Additionally I checked the same configurations with smoothness indicator 2 / formula (38), as well as Fig. 5 d-i, to which I don't have reference values.

The results seem fine, too.