Improve documentation on fluids / physics table

[haraldkl]

The documentation on the fluids/physics table is currently not sufficiently good to allow new users to easily grasp what is needed and how material parameters for the simulation are to be set.
I believe, the explanation currently is found in examples, and it does not provide the necessary information. Rather, there is a lot of text on the SI System, which isn't really needed there. It also says that physical and LBM units have to be provided, though a conversion between them is possible.

An explanation on the specific variables that are actually read by Musubi and the logic behind them and their necessity, aswell as guides on how to obtain them from physical units or similarity numbers, that users would expect.

[haraldkl]

Now that we have #14 merged, could we address this issue?

[haraldkl]

This was fixed by #26.