Reexport alga crate

As alga traits appear on public interfaces and mentioned regularly (ex: https://www.nalgebra.org/generic_programming/#dimensional-genericity) I'd suggest to re-export the whole alga crate. nalgebra seems to rely on it a lot and it is not an implementation detail any more.
Clients have to add alga to the cargo.toml and they have to watch for the version used by nalgebra (especially in early dev stages where we tend to use the latest version from everything).

Unfortunatelly it'is a lack/bug/missing feature/design decision/design flaw (not sure which is the correct word) of the rust module system that it allows to have traits from privatly imported creates to appear on public interface and does not enforces reexports.

(see also #372)