allow to skip through time with a slider widget in CML path plotting function

[cchwala]

I hava a prototype for how it works to use a slider for skipping through time when coloring CML path lines in matplotlib, see the code that starts here.

It might however be better to do this outside of the existing plotting function, i.e. by updating the lines that are returned. This way we could update also other matplotlib object, e.g. points from plotting gauges and a pcolormesh grid from plotting radar data. This could then be done with one time slider to skip through all data simultaneously. The time stamps could snap to the closes one if the data has different time spacing.