[mlpack] Splitting the optimization framework into its own library: "ensmallen"

[Ryan Curtin]

Hello there everyone,

After a lot of discussion, we've decided that splitting the optimization
framework found in src/mlpack/core/optimization/ into its own library.
This will help make the optimization framework more visible and easy to
use from other projects that aren't mlpack.

We've decided to call the library "ensmallen", and the repository will
live at https://github.com/mlpack/ensmallen.

Of course, the release is not ready yet, and I'll send another email
once we've released version 1.0.0 of ensmallen.  If you've contributed
to the optimization framework in the past, your name will be on the list
of credits for ensmallen, too.

Anyway, this email is just to let you know!  I hope that when it is
ready, you'll find it useful.

Thanks and have a great day!

Ryan

-- 
Ryan Curtin    | "What? Facts?"
ryan at ratml.org |   - Joe Cairo