Improve the truncation of the "domain_centre in generate_mesh

### Are there any linked Issues or Pull Requests?

No

### Brief description

The mesh file contains coordinates for the **domain_centre** and **north_pole** for planar meshes in their _"constructor_inputs"_ variable, these are truncated to the second decimal. These variables are read by some regional workflows to build mesh again for different purposes.

A second decimal truncation is fine for **north_pole** but not for **domain_centre**, as some our LFRIC regional domain should match some of the boundaries of previous operational UM domains for coupling or upstream/downstream applications, thus requiring more precision.

### Further details of the issue.

This issue will change the truncation of the domain_centre where written to the mesh's _"constructor_inputs"_ variable to include 6 digits instead of two (changing **F10.2** to **F10.6**, enough for longitude (must be always positive for other tasks) and latitude.