Get rid of SE_NodeBased grid type

[bartgol]

That grid type is basically implementing the same thing as MeshFree. I propose to make SEGrid to always be what we now call SE_CellBased type, and unify MeshFree and SE_Node_Based. In particular:

• GridType should contain Undefined, SE, Point, with the latter replacing MeshFree.
• SEGrid should just be of type SE, and always store elem/gauss-pt information (like the SE_CellBased currently does)
• Rename SimpleGrid into PointGrid, reflecting the type Point in GridType.

This will help keep code duplication low, and the code clearer and cleaner.

[jeff-cohere]

@bartgol : this is finished, right?

[bartgol]

Done in #716