RF: Tidy up

[scottclowe]

• Don't need to import builtins.
• Can always assume multiprocessing is available as it has been part of the standard library since Py 2.7.
• Use os.path.join for building path to test data.

[codecov]

Codecov Report

Merging #163 (9870c1d) into master (2c54b62) will increase coverage by 0.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
+ Coverage   92.38%   92.68%   +0.29%     
==========================================
  Files           9        9              
  Lines         788      779       -9     
  Branches      158      158              
==========================================
- Hits          728      722       -6     
+ Misses         32       29       -3     
  Partials       28       28              

Flag	Coverage Δ
unittests	92.55% <100.00%> (+0.29%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
fissa/ROI.py	78.26% <ø> (-0.47%)	⬇️
fissa/readimagejrois.py	91.38% <ø> (-0.09%)	⬇️
fissa/roitools.py	88.98% <ø> (-0.10%)	⬇️
fissa/core.py	95.91% <100.00%> (+1.11%)	⬆️

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