quadrature: Put zeros in interp matrix where appropriate.

This fixes the MLSDC examples.  Guess how long that took to track down?

Signed-off-by: Matthew Emmett <memmett@gmail.com>
Merge pull request #36 from memmett/feature/fix-interp

Fix interpolation matrix.
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>