add implementation of From trait for SMatrix<->DMatrix, SVector<->DVector

[Lishen1]

[Andlon]

Thanks for the PR @Lishen1, I think this kind of functionality is sorely needed in nalgebra. Converting between different storages often requires different approaches depending on what needs to be done. A more unified approach to conversion would be great.

That said, I think the current approach is a little bit too narrow. For example, it implements directly on DMatrix<T>, which excludes e.g. DVector<T>.

I wonder if we instead could do something like From and TryFrom impls that just forward to the underlying Storage, and then implement the proper conversion routines for storages instead. Essentially, we'd have TryFrom<VecStorage> for ArrayStorage and From<ArrayStorage> for VecStorage> assuming that dimensions satisfy something like DimEq (or ideally SameNumberOfRows/SameNumberOfColumns if possible, for more granular type mismatch errors).

Do you have any thoughts on this?

[Lishen1]

probably we can combine implementation for vector and matrix into one implementation like:

impl<'a, T: Scalar, const R0: usize, const C0: usize, R1: Dim, C1: Dim>
    From<&'a Matrix<T, R1, C1, VecStorage<T, R1, C1>>>
    for Matrix<T, Const<R0>, Const<C0>, ArrayStorage<T, R0, C0>>
{
    fn from(_m: &'a Matrix<T, R1, C1, VecStorage<T, R1, C1>>) -> Self {
    ...
    }
}

i don't know how we can use SameNumberOfRows/SameNumberOfColumns since R1: Dim, C1: Dim both can be Dyn and we can't proof shape match in compile time. anyway, i'll think how to improve implementation with yor suggestions