Handle CubeLists in all operators that operate on Cubes

[jfrost-mo]

What problem does your feature request solve?

Currently there is not a good way to run operators that only take a single cube over all cubes in a CubeList. For example, regridding. This will then be able to read_cubes directly on multiple models.

Describe the solution you'd like

These operators should be expanded to handle a CubeList. This should be done by looping over the cubes inside the operator, before returning another CubeList of the processed cubes.

Describe alternatives you've considered

We could add another operator to loop the existing single cube operators. It would have a signature similar to this:

- operator: misc.apply_to_all
  operator_to_apply: regrid.regrid_onto_xyspacing
  args:
    xspacing: 0.5
    yspacing: 0.5

I'm tempted to avoid it however, as it makes recipes harder to understand, and this functionality is part of making an operator more valuable than the underlying iris code. Looping within the operators also unlocks opportunities, like caching the regridder in #720.

Individual tracking issues:

• collapse.collapse should be able to handle a CubeList #540
• aggregate.time_aggregate should be able to handle a CubeList #911
• transect.calc_transect should be able to handle a CubeList #912
• misc.addition should be able to handle a CubeList #913
• misc.division should be able to handle a CubeList #914
• misc.multiplication should be able to handle a CubeList #915
• misc.subtraction should be able to handle a CubeList #916
• plot.plot_histogram_series should be able to handle a CubeList #917
• plot.plot_line_series should be able to handle a CubeList #918
• plot.plot_vertical_line_series should be able to handle a CubeList #919
• plot.scatter_plot should be able to handle a CubeList #920
• plot.spatial_contour_plot should be able to handle a CubeList #921
• plot.spatial_pcolormesh_plot should be able to handle a CubeList #922
• regrid.regrid_to_single_point should be able to handle a CubeList #924
• convection.cape_ratio should be able to handle a CubeList #925
• convection.inflow_layer_properties should be able to handle a CubeList #926
• regrid.regrid_onto_xyspacing has incorrect return type signature #923

[jwarner8]

@jfrost-mo I will probably create a branch to update the regrid operator to handle a cubelist (realise this issue is for operators more generally). Reason for this is that to implement regrid caching, for one of the functions I need this capability (see comment in #720). Mentioning here to avoid duplication in case you start this work on all operators including regrid!

[jfrost-mo]

That sounds good. We probably want a few different branches for the different operators that need conversion to make reviewing easy.