Refactor / experiment init

[WilliamHPNielsen]

In the spirit of #1323, this PR proposed to do the same for the Experiment class

Changes proposed in this pull request:

• Refactor the __init__ of Experiment so that no _new method exists
• Add a test for the new behaviour
• Fix two broken properties of the Experiment (started_at and finished_at)

@astafan8 @QCoDeS/core

[codecov]

Codecov Report

Merging #1332 into master will increase coverage by 0.02%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master    #1332      +/-   ##
==========================================
+ Coverage   72.55%   72.57%   +0.02%     
==========================================
  Files          74       74              
  Lines        8569     8577       +8     
==========================================
+ Hits         6217     6225       +8     
  Misses       2352     2352

[astafan8]

awesome work! just some minor things to do in my opinion, see comments

[astafan8]

actually, there was no problem with the _new method. It has _, hence noone should call from outside of the class. But that was exactly the problem :) anyway, of you feel like, please don't hesitate to encapsulate the else-clause part of the logic into a private method in order to increase readability of the __init__ method.