Add site ex1b to meto in rose-stem

### Are there any linked Issues or Pull Requests?

_No response_

### Brief description

Small LFRic_atm jobs can run simply on ex1a or ex1b. This is currently controlled on the command line by the user: `-S USE_GENOA=true`. However for large models, such as the C896, the Genoa processor is sufficiently faster than Milan that it is causing network slowdown issues. These can be handled with network environment variables that run in software, rather than hardware. These issues will be ultimately resolved when slingshot is upgraded from SHS12.02 to SHS14.0.x but it is uncertain when the upgrade will take place. The environment settings change will enable the networking to use software mode, but this uses extra memory and for some large lfric_atm jobs will cause ex1a (Milan) to run out of memory. ex1b (Genoa) has more memory and can run in software match mode. The cleanest way to handle the different configurations for the different machines is to have an ex1b machine configuration at the meto site separate to ex1a. They are different machines.

### Further details of the issue.

_No response_