Merge refactor/grids (missed delete of a note) Changes from previous merge: * `EquidistantGrid` is now only a 1D thing. * Higher dimensions are supported through `TensorGrid`. * The old behavior of `EquidistantGrid` has been moved to the function `equidistant_grid`. * Grids embedded in higher dimensions are now supported through tensor products with `ZeroDimGrid`s. * Vector valued grid functions are now supported and the default element type is `SVector`. * Grids are now expected to support Julia's indexing and iteration interface. * `eval_on` can be called with both `f(x,y,...)` and `f(x̄)`.