New Grid API

[ddudt]

Resolves #1840

Restructures and generalizes the Grid classes. The new ABC AbstractGrid that all other grids inherit from does not assume a particular coordinate system. The new ABC AbstractGridFlux is specific to ($\rho,\theta,\zeta$) coordinates and is the parent class of all existing grid classes in flux coordinates. Most of the changes involved moving existing code into one of these new ABCs.

The goal is to allow for new grid classes in the future that represent other coordinate systems, without changing the existing public API.

To-Do:

• Ensure tests pass
• Generalize Basis dimensions (no references to flux coords)
• Include isinstance(grid, AbstractGridCurve) checks wherever appropriate
• Add 1D grids for Curve objects
• Add 2D grids for FourierRZToroidalSurface objects
• Alias flux grid class names like Grid -> CustomGridFlux, etc. (do not deprecate)
• Update docs/dev_guide/notebooks/grid.ipynb
• Utility function for LinearGrid._create_nodes?