[WIP] Adding Ewald+SPME longrange force solvers

[sfogerty]

Creating a new longrange force class, derived from the Force class.

Both Ewald and smooth particle mesh Ewald (SPME) are included - note that the Ewald Sum does not use any FFTs or meshes, only SPME.

Closes #25

[sfogerty]

Still WIP. I hope to get the basics laid out, and have everything successfully build before asking for some review.

[streeve]

Let me know when you want me to look through this in detail.

At first glance, you can remove the ExaMiniMD license from the new files you've added, consider a separate folder for longrange_force_types, and make sure using longrange is fully optional.

The other thing to consider is deriving the longrange classes from Force, rather than a new base. We could share the neighbor list and energy compute more easily that way

[streeve]

Here's what I noticed in a first pass; mainly I'd like to separate the force compute and the comm, potentially the energy and the force. Getting all of this simplified should make the mesh methods much easier