Fix get_context of multiprocessing for windows compatibility

[DrTaDa]

Fixes issue #453 ?

[AurelienJaquier]

Then windows users might encounter this error: #424

[codecov-commenter]

Codecov Report

Merging #454 (120b1e9) into master (4227007) will decrease coverage by 1.24%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
- Coverage   87.46%   86.23%   -1.24%     
==========================================
  Files          50       50              
  Lines        4325     4330       +5     
==========================================
- Hits         3783     3734      -49     
- Misses        542      596      +54     

Impacted Files	Coverage Δ
bluepyopt/ephys/protocols.py	60.99% <50.00%> (-11.94%)	:arrow_down:

... and 4 files with indirect coverage changes

[AurelienJaquier]

Maybe at least give a warning telling the user that things might break if s/he uses non-default dt value?