- Some important clean-up of FEM data structures

- Added reference length to Grid
- Removed old Grid::Type enumerator (use dynamic_cast instead)
- Added GridGLL to contain functionality for FEM methods on any grid
- Moved DxBasis1D and Stiffness1D from HorizontalDynamicsFEM to GridGLL
- Removed all reference to the cubed-sphere from HorizontalDynamicsFEM