Pandas ix fix

[wkerzendorf]

fixing the pandas deprecated use of .ix
convert several python 2 to python 3 within the code.

[codecov-io]

Codecov Report

Merging #886 into master will not change coverage.
The diff coverage is 78.2%.

@@           Coverage Diff           @@
##           master     #886   +/-   ##
=======================================
  Coverage   65.77%   65.77%           
=======================================
  Files          83       83           
  Lines        5922     5922           
=======================================
  Hits         3895     3895           
  Misses       2027     2027

Impacted Files	Coverage Δ
tardis/util/base.py	77.58% <0%> (ø)	⬆️
tardis/io/tests/test_ascii_readers.py	100% <100%> (ø)	⬆️
tardis/io/tests/test_decay.py	100% <100%> (ø)	⬆️
tardis/model/base.py	89.61% <100%> (ø)	⬆️
tardis/plasma/properties/general.py	100% <100%> (ø)	⬆️
tardis/plasma/properties/partition_function.py	92.37% <100%> (ø)	⬆️
tardis/montecarlo/base.py	87.33% <100%> (ø)	⬆️
tardis/plasma/properties/level_population.py	100% <100%> (ø)	⬆️
tardis/io/model_reader.py	97.41% <100%> (ø)	⬆️
tardis/plasma/properties/atomic.py	94.62% <100%> (ø)	⬆️
... and 9 more

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[wkerzendorf]

@chvogl @ssim if one of you could quickly sign off on this.
checks pass.