Check if grid coordinate values have discontinuity

[JiaweiZhuang]

To make issues like #14 easier to discover, xESMF should check if the input grid coordinates are well-defined, monotonically increasing ("smoothly-changing" would be more appropriate), single-tile quadrilateral grids. Basic steps would be:

1. Convert grid coordinate to Cartesian (x-y-z) space so that polar singularities and periodic boundaries can pass the check.
2. Compute grid sizes by something like np.diff()
3. If there's an abrupt change in grid size (criteria to be decided), throw a warning with element index.

Make it an option if it incurs visible slow-down.

[JiaweiZhuang]

Mainly for bilinear and conservative that need to know connectivity. Nearest neighbor methods should be OK with disconnected tiles.