Bayesicfitting

* Clean up and unification of the python doc strings.
* Reran all examples and test harnasses in python 3.7.
* Add random seed to several examples to make them more stable.

* New Classes:
- DecisionTreeModel
A DecisionTree Model (DTM) is mostly defined on multiple input dimensions (axes).
It splits the data in 2 parts, according low and high values on a certain input axis.
The splitting can continue along other axes.
- Modifiable
Interface to define modifiable behaviour of some Models.
- StructureEngine
Engine to modify Models that implement Modifiable
* Introduce Table from astrolib as (multidimensional) xdata
* Some restructering necessitated by the classes above.
* Testcases and examples for the classes above

Add a few more classes, repair some bugs.

First release of BayesicFitting on Zenodo. Earlier versions were available on pypi.org and GitHub.com.