Do not hard-code `OmegaMesh.nc` in Decomp

[xylar]

The name of the Omega mesh file should be read from the config YAML file, not hard-coded:

Omega/components/omega/src/base/Decomp.h

Line 267 in ba2f369

static void init(const std::string &MeshFileName = "OmegaMesh.nc");

Polaris has to make symlinks with this name that should ideally not be necessary.