Add GridMath Operator

[JordanRL]

Create a GridMath class in a new namespace Samsara\Planck\Operator. The job of this class is to take a complete, arbitrarily nested grid and perform calculations on them, such as the distance between two addresses.

It's possible that this could also integrate with samsara/newton to provide distances in units, although that would create a dependency. That likely depends on JordanRL/Newton#10 and JordanRL/Newton#11 however.

[JordanRL]

Probably will need a Euclidean and non-Euclidean GridMath class, since the math will be different for a grid that represents spheres and a grid that represents space.