Potential refactoring of the backends into a solver class and a backend class

[semi-h]

We can do a refactoring so that the backend class only provides subroutines that executes kernels. To do this we need to extract a solver class from the current backend class. For example, transeq operation is very specific to Incompact3D algorithm and we can define this under a solver class. We still need child classes for different architectures as the specifics of the operations we carry out differs based on the architecture. However, most operations solver need rely on tridiagonal solvers, so tridiagonal solver kernels can be provided via a backend class that is independent from the solver.

This could be useful when we want to extend the capabilities of the new framework such as adding low mach number support. The low mach number solver can specify its own preferences and execute transeq in a slightly different way.