QA/QC wishlist

Here is a list of things that prevented the ESMValTool community from working with CMIP6 data that we would like to avoid in CMIP7. This is summarized from the [fixes in ESMValCore](https://github.com/ESMValGroup/ESMValCore/tree/main/esmvalcore/cmor/_fixes/cmip6).

# Can be checked per file

## Can be checked without accessing data
- Parametric vertical coordinates were unusable: https://cfconventions.org/cf-conventions/cf-conventions.html#parametric-v-coord. I have not been able to find a single CMIP6 file that follows the CF conventions for these coordinates. Could this be a bug in CMOR?
- Wrong variable name for coordinate (example: variable name/[out_name](https://github.com/PCMDI/cmip6-cmor-tables/blob/087fe45d21c082e28723e0f930e4266abe91b853/Tables/CMIP6_coordinate.json#L823-L831) `latitude` instead of `lat`)
- Wrong or missing `standard_name`, `long_name` attribute for coordinate
- Invalid value for `parent_time_units` attribute (for example: `days since 1850-01-01-00-00-00`)
- Missing `positive` attribute for vertical coordinates
- `_FillValue` or `missing_value` attribute dtype does not match data dtype (for example: data dtype is float32 and missing value dtype is float64)
- Missing scalar coordinate (e.g. [`height2m`](https://github.com/PCMDI/cmip6-cmor-tables/blob/087fe45d21c082e28723e0f930e4266abe91b853/Tables/CMIP6_coordinate.json#L680-L702), `typeland`)
- Missing coordinate bounds (for example for the `latitude` coordinate)

## Need to read data
- Non-monotonic coordinate value or bounds
- Units attribute does not correspond to the actual data (e.g. data in degrees Celsius but units say Kelvin, data is a fraction but units say percentage)
- Data uses a different `_FillValue`/`missing_value` than declared in the attribute

# Can be checked for collections of files

## Can be checked without accessing data
- Different `calendar` attribute on the time coordinate of files from the same dataset
- Differences in `long_name` for horizontal index coordinates for 2D grids within the same dataset

## Need to read data
- Slightly different coordinate and bounds values within the same dataset