Clarify documentation of {CombiTimeTable, CombiTable1Ds, CombiTable1Dv}.smoothness

[AHaumer]

Is it possible to explain - maybe with links to further reading - the different spline interpolations a little bit more in detail: 2 - Akima 4 - Fritsch-Butland 5 - Steffen 6 - Modified Akima Since this is the same for different tables and the correct documentation of all the usefull features of all tables is important, it could be an idea to have a section in a User's Guide, and the different blocks' documentation links to that section.