newunit specifier for opening files

[SeanBryan51]

Maybe a good place to start introducing the newunit specifier?

CABLE/src/offline/cable_driver_init.F90

Lines 98 to 100 in 529e83c

	OPEN(10, FILE=CABLE_NAMELIST, STATUS="OLD", ACTION="READ")
	READ(10, NML=CABLE)
	CLOSE(10)

Suggestion:

    OPEN(newunit=unit, FILE=CABLE_NAMELIST, STATUS="OLD", ACTION="READ")
    READ(unit, NML=CABLE)
    CLOSE(unit)

with unit being an integer to be declared above.

This is safer and easier to read (here, 10 is a magic number)

Originally posted by @micaeljtoliveira in #440 (comment)

[SeanBryan51]

@Whyborn may be relevant for #438?

[micaeljtoliveira]

This is definitely F2008, so might not be acceptable, given the current coding standards.

[Whyborn]

Yea I think it's a good idea to help bring CABLE into the 2010s, although I don't think it's that relevant to that PR. Could extend the scope of the PR to being a modernisation of reading namelists.

[Whyborn]

I don't think we're meeting any standards at the moment, so I wouldn't worry about it being a F2008 addition.

[micaeljtoliveira]

@ccarouge mentioned that we need to follow the JULES standards, and those say Fortran 95/2003 only 😢

[Whyborn]

Are you sure? This conflicts with conversations we had in slack yesterday, where the LFRic Coding Standards were the standards we wanted to follow. Based off the comments and the crosstable linked in this forum post, I thought we came to the conclusion that standards/features up to F2008 were allowable.

[micaeljtoliveira]

Okay, chatted with @ccarouge and LFRIc standards are the way to go 🎉

[Whyborn]

I have made a start on this issue, including the NEWUNIT specifier for all namelist OPEN instances as part of improving namelist error handling #438. I can extend this to all occurrences where we open a new non-netCDF file? A bit out of scope of the original PR, but might be worth making these changes all in one go