Python3 version of TARDIS

[wkerzendorf]

This PR enables the use of Python 3 for TARDIS. It also upgrades the versions of several of the underlying packets. The way to create a usable env for this version is to use the tardis_env3.yml file.

We have also had several other smaller changes. Most notably there is now a tardis.constants module that imports all the constants from astropy. This PR currently still uses the constants version of astropy13 (using the mechanics in astropy31).
This PR also has the side-effect that several known installation and testing problems disappeared.

All tests currently pass and the spectra look the same within the noise.

There are still a few documentation changes to make.
@chvogl @ssim I need one or both of you to sign-off on this PR.

(@yeganer your dream has finally come true)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@89270c2). Click here to learn what that means.
The diff coverage is 77.31%.

@@            Coverage Diff            @@
##             master     #879   +/-   ##
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  Coverage          ?   65.77%           
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  Files             ?       83           
  Lines             ?     5922           
  Branches          ?        0           
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  Hits              ?     3895           
  Misses            ?     2027           
  Partials          ?        0

Impacted Files	Coverage Δ
tardis/montecarlo/setup_package.py	0% <0%> (ø)
tardis/plasma/setup_package.py	0% <0%> (ø)
tardis/analysis.py	0% <0%> (ø)
tardis/simulation/setup_package.py	0% <0%> (ø)
tardis/gui/datahandler.py	0% <0%> (ø)
tardis/plasma/base.py	60.26% <0%> (ø)
tardis/io/config_reader.py	81.9% <0%> (ø)
tardis/gui/widgets.py	0% <0%> (ø)
tardis/plasma/standard_plasmas.py	82.89% <0%> (ø)
tardis/plasma/properties/ion_population.py	90.69% <100%> (ø)
... and 18 more

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[ssim]

Good job - Py3 here we go!?