advection_diffusion: Add 'as_dvector' and use references.

This shortens up a bunch of code and is (hopefully) easier for newcomers
to read.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Silence a bunch of compiler warnings.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Merge branch 'release/v0.1.0' into feature/by-reference
vector: Move as_vector to vector.hpp as suggested by Ruprecht.

This also overrides for the const case, as suggested by Klatt.

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Merge pull request #35 from memmett/feature/by-reference

Go back to using references in AdvectionDiffusionSweeper.