plot_multiple kwargs cannot contain labels key

core.result.plot_multiple() allows for **kwargs which are all passed to plot_corner(). However since one of the inputs of plot_multiple() is labels, if kwargs contains labels as one of its keys it will be primarily understood as the input for plot_multiple() and won't be passed to plot_corner() hence making it impossible to define one's own latex labels for the corner plot.

I reckon there are two solutions for this:

1. rename plot_multiple() 's labels input to result_labels to get rid of the ambiguity.
2. add a corner_labels input which would set kwargs['labels'] = corner_labels (if not None).

In my opinion the first solution is better but I can imagine it would cause some backward compatibility issues.