Reworked Configuration Namelist Access API

[mo-rickywong]

PR Summary

Sci/Tech Reviewer: @allynt
Code Reviewer: @MatthewHambley

This PR is to allow users more direct access to the namelist configuration values from
Fortran object (rather than global module scope) while maintaining it's read-only nature.
e.g.
modeldb%config%<MyNamelist>%MyNamelistMember()

This PR is linked to LFRic-core trac ticket #4702, which provides more details on the change itself

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trac.log

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[MatthewHambley]

I notice that across the board you assign namelist values to variables before using them. Can you confirm this is a choice you've made and that, as functions, they can be used in-line. e.g. call something(value, modeldb%config%my_namelist%other_value().

Yes this is a choice, the values can be accessed in-line. I have extracted them as the variables existed and shortens the actual code so it appears cleaner.

[MatthewHambley]

Not too many changes, some of which may be postponed to later sets.

[MatthewHambley]

There's still much to be done to tidy things up but this gets the basic functionality in place.

[mike-hobson]

From a code owner's point of view, this seems fine.