Run tests in parallel

[GateBuilder]

This PR introduces changes to run test suit in parallel with pytest-xdist.

@jenshnielsen

[GateBuilder]

@jenshnielsen Nodes are crushing :(

[GateBuilder]

Instead of auto I suggest to use 4 cores for starters.

[GateBuilder]

Travis runs on 2 cores at auto mode :) (also pipelines)

[codecov]

Codecov Report

Merging #2040 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2040      +/-   ##
==========================================
- Coverage   71.43%   71.43%   -0.01%     
==========================================
  Files         149      149              
  Lines       20065    20063       -2     
==========================================
- Hits        14333    14331       -2     
  Misses       5732     5732              

[jenshnielsen]

there are still references to math within field_vector.rst

[astafan8]

was it really necessary to move the field_vector to math_utils?

[GateBuilder]

@astafan8 We were getting ImportError: cannot import name 'log' from 'math' (/home/travis/build/QCoDeS/Qcodes/qcodes/math/__init__.py), which should be imported from elsewhere.

[jenshnielsen]

an alternative solution would probably be to avoid running the tests from within the qcodes source directory. Moving math is unfortunate as it is a breaking change but it seems like it should never have been called math in the first place

[jenshnielsen]

this seems to be the problem pytest-dev/pytest-xdist#421

[GateBuilder]

@jenshnielsen let us merge this! nightmare. :)

[jenshnielsen]

Please write a note in the changelog that the math module has moved

[jenshnielsen]

There are some issues with the code coverage that we need to fix

[jenshnielsen]

@astafan8 Do you want to have a final look?