Bringing TerraNeo app to master

This issue collects subtasks that need to be tackled to step-by-step migrate the TerraNeo to the master branch.

State:

As of writing this issue there are multiple branches with different states of the 3d_convection app.

Agenda:

Rebuild 3d_convection app (maybe rename) only from features that are in the master branch. If some feature is missing - add it to master first. This is just a proposal, but possibly the most straightforward way to accomplish this.

For example:

• branch away from master
• copy single feature from older branch
• create a small test
• merge it back to master
• continue

Subtasks

• separate RHS assembly from operator classes (@dascoli told me there are some old functions from @MarkusWiedemann that could be reused)
• sort out how/where to apply all the projections (free-slip and pressure) in the solvers @burkhart @dascoli
• bring solvers to master
• generate all required operators and have them in master
• use new generated operators and build composites where necessary
• modularization of initialization functions for temperature, viscosity, etc. (could be directory in src/terraneo)
• modularization of I/O functions (readers for file formats etc.)
• reduce/streamline number of FE function in TerraNeo app, maybe add std::map< std::string, std::shared_ptr< FunctionType > > (see here)
• ensure that the TerraNeo app will work for variable mesh-width in radial direction (see #259 (closed) for details)

Please feel free to contribute to this task including discussion: @IsabelPapanagnou @burkhart @dascoli @pponkumar @mohr