Add a cube to cubelist operator

[daflack]

What problem does your feature request solve?

For recipes that require multiple cubes to be entered into the same function, we need to collate them into a cube list to allow easier reading and collating.

Describe the solution you'd like

Operator should take multiple cubes as separate arguments and combined them into a single cube list.

Describe alternatives you've considered

[jwarner8]

Links to #721 more generally, and specifically PR #734 for implementation in age of air diagnostic #480. I can't see any branch linked to this issue, so I will create a branch in a second and link to this issue for a basic implementation.