Add approx traits

[mulimoen]

It would be nice if the sparse matrices support the traits from approx, which allows checking whether two matrices containing floats are approximately equal. This to aid in testing of code, both internally in this crate and outside.

[vbarrielle]

That would be nice indeed. I guess there would first be a structure check, and then a check to see if the norm of the difference between the data vectors is small enough.