sweeper: Add `converged` virtual function.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
encap: Add virtual norm0 routine.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
imex: Add concrete `residual` method.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
encap: Provide default `converged` method.

The default computes the maximum (across all nodes) absolute residual
norm as provided by the sweeper, and tests it against
`abs_residual_tol`.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
encap: Add `set_residual_tolerances` method.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
encap: Fix doc.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
mlsdc: Add support for convergence checking.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
tests: Add test to make sure residual tolerance is working.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
imex: Fix `residual`.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
mlsdc: Add `converged` flag.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
mlsdc: Rearrange (again) to follow pfasst.hpp.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Merge pull request #108 from memmett/feature/residual2

Add iteration control mechanism and provide default based on residuals (2nd try).

LGTM. Merging it.