Create a CSET community environment at the Met Office

### What problem does your feature request solve?

While CSET can be installed through conda-forge, it takes some time to download the packages. Additionally there is always the risk that a new package version breaks something.

### Describe the solution you'd like

To make CSET as easy to use as possible we should set up a pre-made software environment that people can `module load` with ease.

### Describe alternatives you've considered

While we could create a conda environment in a personal space and ask people to activate that, it is harder to use. Community environments are supported at the Met Office, so we should use them.