public recursive subroutine fillHelperElementsFineToCoarse(scheme, general, physics, iLevel, maxLevel)
Recursively fill all the helper elements (i.e. ghost, halo) with valid
information from the fluid elements.
This step is required before each run of the simulation. It would be
possible to fill also the helper elements with the initial conditions.
However, we are only able to fill the fluid elements with valid data
(restart files have no information about the helper elements)
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