Solvers and runs

Linear solvers and preconditioners, Newton, time stepping, updates between intervals, checkpoints, the progress log and the settings of a run.

• Linear solvers and Newton: src/solvers/linear_solver.jl, src/solvers/krylov.jl, src/solvers/block_preconditioners.jl, src/solvers/fluid_schur_preconditioner.jl, src/solvers/newton.jl
• Time stepping: src/solvers/time_stepping.jl, src/solvers/legs.jl
• Updates between intervals: src/solvers/interval_updates.jl, src/solvers/transport_update.jl
• Checkpoints and progress log: src/solvers/checkpoint.jl, src/solvers/progress_log.jl
• Run settings: src/settings.jl