Handle CubeList in line plotting operator

[Sylviabohnenstengel]

Fixes #918

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[Sylviabohnenstengel]

addresses issue #918

[github-actions]

[jfrost-mo]

The right idea, the big change needed is that we can just return the original input data, as this function doesn't modify it.

Otherwise just some comments around variable naming and documentation.

[jfrost-mo]

As this operator doesn't modify the data, we can just return the original thing we were given, rather than having to store it.

[jfrost-mo]

I don't like these runtogetherwords, as I read that as "top lot". Python code tends to use snake_case, where the words are separated with underscores.

The operators we already have that take either a Cube or a CubeList call this first arguments cubes, so perhaps we should continue that.

[jfrost-mo]

As above, we can skip this if we return the original cube(s).

[jfrost-mo]

Oops, I've realised I've reviewed this PR wrong. While this is the correct transformation for most of the operators that need converting to handle a CubeList, this particular one is a bit special.

When we give the line plot operator multiple cubes, I suspect we are wanting multiple lines.

Apologies for not being clear in the ticket.

[jfrost-mo]

The main think that need to be done here now is to take the cubes into the internal plot_and_save function so each can be plotted as lines on the same axis.