Enhance plot functions

[LeFrit]

The UncertainFunction.plot() method could use some updates. I have recently built my own function which is accessing _mcpts externaly (feels kinda dirty) an is using seaborn. The results are quite nice though. Also, returning the ax object for further usage would be great.

I'll try to fork the project and work on the suggestion myself - I'm kinda new to the GitHub and OpenSource world, so let's see how this will work out.