Changed minor things like idx2ind is now coord2idx for clarification
to make deailng with poitners a bit nicer i've added .coeff and
.coeffRef methods so you can do ptr->coeff() vs (*ptr)() which is uglier
in my opinion.
Made the factory maintain a bounding box instead of an origin because
the bounds of the staggered grid are a bit more interesting/useful and
it's only EmbedDim*sizeof(Scalar) more data.

Added two modes of looping over the grid: one where the coordinates
aren't important and another where the coordinates are important.

There's now commented code that will become an auomatic resizing / grid
managing system.