TS: add TSSetTransientVariable (alias for DMTSSetTransientVariable)

Requested-by: Hong Zhang <hongzhang@anl.gov>

TS: add support for transient variables in BDF

This feature enables conservative time integration even when using
nonconservative state variables. The formulation, which requires only
the definitio

TS: add support for transient variables in BDF

This feature enables conservative time integration even when using
nonconservative state variables. The formulation, which requires only
the definition of transient (conservative) variables C(U) as a
function of the state variable U, is appropriate for linear multistep
methods for ODE and DAE.

This technique could be extended to Runge-Kutta methods by creating an
index-1 DAE by augmentation with the equation C = C(U). The stage
equations in diagonally implicit methods generally ask to solve for Y:

F(Y, alpha Y + Z) = 0

where Y = [C, U] and Z is known from prior stages. With the augmented
equation, this decouples to

C - C(U) = 0
f(U, alpha C + C_z) = 0

This commit includes an example demonstrating loss of conservation in
chain rule primitive-variable IFunction, while preserving it for
pure-conservative variables and the transient variable formulation.

Resolves #547

Requested-by: Gautam Bisht <gautam.bisht@pnnl.gov>
Thanks-to: Hong Zhang <hongzhang@anl.gov>

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TS+ConvEst: Made suggested improvements
- Renamed initialGuess to initialConditions
- Made refinement factor usable settable, but still only support 2 for
space

ConvEst: Generalize to use TS as well
- Now the solver is a PetscObject
- ts_convergence_estimate calculates the order for a TS method
- Added tsconvest.c
- Added initial guess and error methods that

ConvEst: Generalize to use TS as well
- Now the solver is a PetscObject
- ts_convergence_estimate calculates the order for a TS method
- Added tsconvest.c
- Added initial guess and error methods that can be overridden

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