Variable-implicit solver and dualtime_advance() exposure
- Add variable-implicit implicit BDF method, which uses the diagonal of the Jacobian matrix
- Add corresponding sensitivities to ODE test cases and the general BDF residual expression
- Modify implicit integrators to expose internals of each physical time step
- All dual time tests now use the new framework, running both the physical and dual time loops
- All tests but the heat equation use an initialization tree, required for sensitivity propagation
- Modify integrators to use expressions for the physical and dual time step
- Modify regression test of fixed-point BDF-1 to check convergence rate
- Add regression test for variable-implicit that checks both solution accuracy and convergence rate
- Rename several dual time integrator members and methods
- Add Matlab test for exp decay problem with fixed-point and variable-implicit