Implement SVD by divide-and-conquer

[nlhepler]

Obviously I've made a lot of arbitrary choices here (such as the name. SVDDC? SDD? erk).

[nlhepler]

I could for instance merge this with the existing SVD_ trait, and change the trait to provide another function (svddc, or sdd). This may be preferred.

Or alternatively a more complicated way to invoke the existing svd that switches things around depending.

[termoshtt]

I'm planning to refactoring these basis of Lapack traits (SVD_ and others) into single trait, but it should be another PR. Your implementation is enough match to current impls, and I will accept it.

[nlhepler]

Thinking forward to rand_svd, perhaps a configuration like the following is suggested?

#[repr(u8)]
pub enum SVDFlag {
    Full = b'A',
    Some = b'S',
    None = b'N',
}

pub enum SVDConfig {
    SVD { u: SVDFlag, vt: SVDFlag },  // update gesvd wrappers to support 'S'
    SDD { flag: SVDFlag },
    Rand { k: usize, l: usize, n_iter: usize },
}

impl SVDConfig {
    pub fn svd(u: SVDFlag, vt: SVDFlag) -> Self {
        SVDConfig::SVD { u, v }
    }

    pub fn sdd(flag: SVDFlag) -> Self {
        SVDConfig::SDD { flag }
    }

    pub fn rand(k: usize, l: Option<usize>, n_iter: Option<usize>) -> Self {
        let l = l.unwrap_or(k + 2);
        let n_iter = n_iter.unwrap_or(2);
        SVDConfig::Rand { k, l, n_iter }
    }
}

[termoshtt]

Thinking forward to rand_svd, perhaps a configuration like the following is suggested?

this seems good. I prefer svd_rand instead of rand because a.svd_rand(...) is better identifiable than a.rand()

[termoshtt]

The unification of SVD flags as you suggested in comment should be done after introducing random SVD, and another PR. I merge this PR as it is. Thanks a lot!