Add features required for the migration of rusty-machine to nalgebra

See [that issue](https://github.com/AtheMathmo/rusty-machine/issues/199) from rusty-machine.
So far, the following features are missing from nalgebra:

- [x] Simplify the construction of `DVector` by addressing https://github.com/rustsim/nalgebra/issues/377 This will require breaking changes.
- [x] Add statistics functions: variance, mean.
- [x] Add more general norms by addressing https://github.com/rustsim/nalgebra/issues/258
- [x] Add row and column iterators.
- [x] Add `.select_rows(iterator)` and `.select_columns(iterator)` to extract a subset of rows or columns from a matrix.
- [ ] Allow the concatenation of rows or columns by addressing #446.
- [ ] Document all the added features, including doc-tests.

More element may be added to this list as I study the codebase of rusty-machine.