Make display generical for Omatrix

[Nateckert]

In the previous version, the test added in tests/core/matrix.rs failed to compile.

This version remove the dependency to the Allocator<usize>, so that the display implementation does not depend of the allocator of nalgebra, making it generical for OMatrix.

Not relying on Omatrix<usize, R, C> implies that some computation have to be done twice.
However, the performance hit is only in Display, which I find acceptable (the use case will be for debugging/writing logs)

The difference with the previous version is that val_width is called twice per entry instead of once.

[sebcrozet]

Thanks!