Strict axis checking in PrePARE

[aradhakrishnanGFDL]

PrePARE to check the following.

-If the variable in the input datasets confines to the CMOR standards (and using CMIP6 Model Output Requirements from Karl et al) in terms of axis and dimensionality. This includes an optional high-level basic checks, with the following being the wish-list/use-cases:

1. checking 2D, 3D axis names ;
2. singleton dimension names ;
3. model level
4. pressure level fields
5. unstructured grids, corresponding coordinate attributes/axis.

Recording this as an issue after an offline conversation with Karl, Charles.