Using variables from FCI grid files in the BOUT input

[totork]

I am currently trying to set up 3D-MMS tests for very distorted grids in FCI. My solutions are generally dependent on the normalized radius rho and the poloidal angle theta. I want to supply the MMS solutions and sources via the input file. The problem here is, that the only coordinates I can use are x,y,z (afaik). Disregarding y here, x and z do not generally coincide with rho and theta. Is there a possibility for me to, for example, add rho and theta to the grid file and use them in the input file to set the MMS solution and source?

[dschwoerer]

One way is to add some variables and then get them in the C++ code with mesh->get("theta") or the like. Then you can implement your expressions in C++.

See this PR for an example using an expression from an input file: https://github.com/boutproject/BOUT-dev/pull/2928/files