revert opening/closing pickle file at each generation

[AurelienJaquier]

Opening/closing files can create a load on the file system if we have multiple optimisations in parallel. Reverting to opening the files once should ease it a little bit

[codecov-commenter]

Codecov Report

Merging #459 (e05b26e) into master (3a28ef9) will increase coverage by 0.03%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #459      +/-   ##
==========================================
+ Coverage   87.43%   87.47%   +0.03%     
==========================================
  Files          50       50              
  Lines        4323     4319       -4     
==========================================
- Hits         3780     3778       -2     
+ Misses        543      541       -2     

Impacted Files	Coverage Δ
bluepyopt/deapext/optimisationsCMA.py	81.88% <0.00%> (+1.16%)	:arrow_up:
bluepyopt/deapext/algorithms.py	100.00% <100.00%> (ø)