Implement Collisionless heat flux into the Braginskii module

[vreville]

This PR adds an option to the bragTDiffusion module. A new keyword is set to either nosat or wcless. With the latter, the heat flux is a combination of the Braginskii heat flux and a collisionless heat flux \propto p v (where p is the thermal fluid pressure and v the bulk velocity). The transition from the Braginskii to the collisionless form is handled through a third parameter alpha (typically an array that depends on the coordinates or the density) while the amplitude of the collisionless heat flux is controlled by a fourth parameter beta, usually constant but can be userdef.

A test and 1D example is given in test/MHD/clessTDiffusion/.
bragTDiffusion tests have been updated to include the new option.

[vreville]

Hum, it looks like the non regression tests from the FargoMHDspherical is trying to compare 16x16x128 cubes with 16x16x64 cubes. I did not touch this file, is this expected ?

[glesur]

Ok, except for the userdef function signature which would benefit from using a single std::vector<IdefixArray3D> as parameter for all of the implementations.

[glesur]

All good, just change std::vector[] by std::vector.at() and we're good to go