Fixes for external grid data read-in method

[fmalatino]

Description
This PR introduces fixes to the external grid data read-in method in generation.py. The method previously lacked a halo update and a call to fill_corners_2d.

How Has This Been Tested?
These changes have been tested outside the CI via the test located in Pace at pace/tests/mpi/ext_grid, as well as with a comparison of the output from a c12 baroclinic run using internally generated grid data and the external data.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[FlorianDeconinck]

So it looks good to me for a code viewpoint. I still don't really understand why we have to transpose things.

@oelbert / @fmalatino : at the very least we should have a docstring in this file or in a readme that spills out your findings

[fmalatino]

So it looks good to me for a code viewpoint. I still don't really understand why we have to transpose things.

@oelbert / @fmalatino : at the very least we should have a docstring in this file or in a readme that spills out your findings

@FlorianDeconinck I do not think we need to tranpose things, this is done to match the layout already used by Pace. I can amend this if we are indifferent regarding what the layout should be.

[FlorianDeconinck]

I can't judge of the scientific validity of this, since I have no idea what comes in, but code wise is ok

[fmalatino]

I can't judge of the scientific validity of this, since I have no idea what comes in, but code wise is ok

@FlorianDeconinck these changes only bring in functionality for reading in longitude and latitude values generated by external sources; scientific validity is assumed to be validated within the generator.