update Gaussian Mixture Model example with pm.NormalMixture (#310)

* create truncated regression example

* delete truncated regression example from main branch

* create truncated regression example

* delete truncated regression example from main branch

* create truncated regression example

* delete truncated regression example from main branch

* initial commit

* update link to pull request in Authors section

* add tag `classification`

* update authorship verbs

* plot through xarray, using XrContinuousRV

* add x axis labels

* add xarray_einstats to watermark, and fix 'classification' as a tag, not a category

Co-authored-by: Benjamin T. Vincent <[email protected]>

Author: Benjamin T. Vincent