Add tests for plots of wallinga_teunis() results

[rebeccanash]

There is now a separate output class for the wallinga and teunis method (PR #203). Need to add plotting functionality for wallinga_teunis object.

[rebeccanash]

Now resolved but need to add tests for plotting

[annecori]

@steffilazerte flagging as mentions tests but not sure this is something you can do. @rebeccanash can you help?

[steffilazerte]

Addressed in #246