Parallel iterators should be supported on uninit-based matrices

[RReverser]

Currently par_column_iter_mut (and its non-mutable variant, although that one is less relevant here) is only implemented for T: Scalar, which makes it impossible to use on matrices created via DMatrix::uninit.

I don't think this bound is necessary, and it makes it impossible to safely fill out an uninitialized matrix in parallel before marking it as initialized.