Adds documentation for LES models and governing equations

[timofeymukha]

See changes in the files.

Maybe just to confirm, for the scalar, we can always have the derivatve outside the viscosity and varying viscosity, right? We need no special treatment for it, as far as I can see.

Also sneaks in a gitignore in the reframe directory.

[muenschm]

Just a minor question, is it possible to have literature sources here as well? If we want to have this as a proper and stand alone documentation, maybe one has to add specify sources (for Vreman for example) as well. Of course, we also have this in the code via the DOIs but to make it complete maybe this would be nice. I can also add this stuff, just don't know if (a) it works in this environment (b) this is supported by the other developers.

[timofeymukha]

I am not sure if one can add an actual bibliography, maybe we can ask in the dev meeting. But we can of course have the DOIs in the documentation as well.

[muenschm]

To my opinion we either write this part for the temperature, then we have to replace s by T in the equation. Or we write this for a scalar s. This also depends a bit on how to continue. If we are going to consider cases where energy equation has to be solved and density variation are considered via Boussinesq approximation in the momentum conservation equation ..... then we also have to expand this for LES ($$\lambda_t$$)

[muenschm]

in the table, the cpldcg-option has to be added for the velocity solve, right?