Release 3.0

[arporter]

As discussed, we need to make a 3.0 release. Mainly to facilitate the new users of the 'code transformation' functionality and to emphasise that the API has changed since the 2.5.0 release.

• 320 nonstandard stop fparser#451
• 448 make public example fparser#450
• FFSL support? (FFSL transport in LFRic depends on a kernel that loops over halo cells #2661)
• Document the OpenMP offload functionality (under https://psyclone.readthedocs.io/en/latest/transformations.html#openmp) and add/extend an LFRic example to show it.
• Update LFRic transformation signature? #2739