Clean up the data model doc and move it to developer documentation

[Steve Mullerworth (stevemullerworth)]

After some discussion, it was suggested that the user documentation be much more focused on technical descriptions of each aspect of the LFRic system and more compact, and that the original data model that described the PSyKAl architecture and how LFRic worked alongside PSyclone should be moved to developer documentation (it was agreed that it may not be an ideal location but it was better than either user documentation or the getting going section).

This pull request makes those changes. It also makes changes to what was called lfric_datamodel:

• In reality, the document focuses on fields and the PSyKAl architecture, and the detail of the core data model that was influenced by the architecture and the requirements of GungHo, so the name has been changed.
• Some of the introductory text has been changed.
• Several extraneous bits of the document have been removed as they are best incorporated elsewhere.
• In working through the document, a few other minor changes have been done.

[Ricky Wong (mo-rickywong)]

This appears as the start of a new page, so there are no "issues above".

[Steve Mullerworth (stevemullerworth)]

Fixed this and one other point where more context was required (this section came from the original data model doc).

[Ricky Wong (mo-rickywong)]

Good, relocation of sections to the developer guide. Majority of issues are more to to with wording and typos rather than actual content

[Ricky Wong (mo-rickywong)]

Suggested change

	grid. The new LFRic atmosphere model uses the GungHo formulation for its
	grid. The LFRic atmosphere model uses the GungHo formulation for its

[Steve Mullerworth (stevemullerworth)]

Done

[Ricky Wong (mo-rickywong)]

Suggested change

	represented by a combination of data points on entities of a mesh and a set of
	represented by a combination of data points on the entities of a mesh and a set of

[Steve Mullerworth (stevemullerworth)]

Done

[Ricky Wong (mo-rickywong)]

"and others" ?

[Steve Mullerworth (stevemullerworth)]

"the LFRic field is a Fortran type
that, among other properties, encapsulates all these aspects of the field."

Changed to this.

[Ricky Wong (mo-rickywong)]

Suggested change

	(c) Crown copyright 2023 Met Office. All rights reserved.
	(c) Crown copyright 2024 Met Office. All rights reserved.

[Steve Mullerworth (stevemullerworth)]

Done (it was a copy of the 2023 version, but, whatevah!)

[Steve Mullerworth (stevemullerworth)]

Responded to all comments. I have chosen not to take the suggestion relating to repeated use of "support" for reasons given, but have changed several of the items in line with requests.