Pyxsteam

* switch to pytest for testing because of 'setup.py test' is deprecated, see https://github.com/pypa/setuptools/issues/1684.

* fix for error in table selection for calculations for density as a function of pressure and enthalpy. Current tests did not indicate this problem - thanks to gonmolina!

* support for Python 3
** handling of math expressions in python 3 fixed problems with some edge cases
** one test still fails: w_ps Speed of sound as a function of pressure and entropy
* fix multiple typos
* linting / cleanup of code
* add constant for absolute zero point
* command line selection for demo program
* new tests for UnitConverter

* change changelog to markdown
* add gitignore
* remove unnecessary files
* reworked setup.py for more sanity
* cleanup and better logging

* fix for typo

* logging capability's