Gather timestepping in NSSolver

[Øyvind Evju]

Raise timestepping form scheme to NSSolver to avoid duplicate code in all schemes. Facilitate generic adaptive timestepping, and further possible expansions in direction of turbulence models and energy equation solvers.

[Martin Sandve Alnæs]

Looking forward to hear what you mean by this, but sounds like major surgery not happening right away?

[Øyvind Evju]

Will not happen right away no.

The timestepping logic is repeated in all schemes, and although this currently is not very sophisticated, with adaptive timestepping (issue #7) that would change.

For this we would need to split the scheme class. I'm visualizing a scheme class with required methods for initalizing matrices and vectors, assemble/reassemble and progress/solve.