Avoid global field during allocation

[lukasm91]

Currently, we allocate FIELDG only to call INIT_FIELDG, which itself is used exclusively for computing the XLON and YLAT arrays. We also deallocate FIELDG within the same routine.

Since the calculation of these arrays is straightforward and does not require any global data, this PR proposes to compute them directly. A further improvement could be to move the XLON/YLAT computation from INIT_FIELDG into a dedicated function to avoid duplicating this code.

[awnawab]

Hey @lukasm91. Thanks a lot for this, it looks like a very sensible change to reduce memory consumption. The CI is currently fubar'd, once #110 is merged I'd like to rebase this over develop and let the CI run through, but essentially this change looks good 👌