559 support external declarations

[Joerg Henrichs (hiker)]

Adds support for external declarations.

[Joerg Henrichs (hiker)]

This is blocked by #558 (it modifies the same file, so will likely cause conflicts otherwise)

[Joerg Henrichs (hiker)]

I've also added support for external attributes (which is from the Fortran grammar point of view different from an external statement), and fixed an issue that subroutines in interfaces where not recorded as dependencies - we clearly lacked a test case there, which I have added.

[Matthew Hambley (MatthewHambley)]

Adds to a previous change. Seems to make sense.

[Matthew Hambley (MatthewHambley)]

I notice that this branch is not at head of trunk. It's probably best if the developer performs the merge.

[Joerg Henrichs (hiker)]

I notice that this branch is not at head of trunk. It's probably best if the developer performs the merge.

Yes, it was blocked by 558 (do you prefer me to mark them as 'draft' instead?). I've merged it all together, should be good to go now