Add superplot that can save lines on 1d slicer plot

[nvaytet]

Example:

%matplotlib widget
import plopp as pp
da = pp.data.dense_data_array(ndim=2)

pp.superplot(da, keep='xx', ls='solid', marker='None')

Fixes #15

[SimonHeybrock]

What does this do to long coords?

[nvaytet]

It's only designed to take in either one or two values, in the case of bin edges (i.e. a slice of a coord with a single index).
But you are right that I should make that more clear, either in the function name or in a docstring