Fixing multiprocessing issue and minor bugfixes

[guillaumegrolleron]

This PR is fixing the issue89 and improves the SPE fit validity flag as asked in issue106

Concerning the multiprocessing issue, I changed the the multiprocessing set up to create child processes from 'fork' to 'spawn', which is the default set up for mac and windows, and recommanded for linux.

I also improved the validity flag for the SPE fit (thanks to @tibaldo).

I also fixed some bugs found by @tibaldo in the photo_statistic module.

[codecov]

Codecov Report

Attention: 153 lines in your changes are missing coverage. Please review.

Comparison is base (270c103) 30.55% compared to head (be6c27f) 30.72%.
Report is 13 commits behind head on master.

Files	Patch %	Lines
.../nectarchain/makers/component/spe/spe_algorithm.py	9.43%	96 Missing ⚠️
src/nectarchain/display/display.py	0.00%	17 Missing ⚠️
src/nectarchain/data/management.py	30.43%	16 Missing ⚠️
src/nectarchain/utils/logger.py	50.00%	9 Missing ⚠️
src/nectarchain/utils/io.py	52.94%	8 Missing ⚠️
...rchain/makers/calibration/gain/photostat_makers.py	33.33%	4 Missing ⚠️
src/nectarchain/makers/core.py	75.00%	2 Missing ⚠️
...chain/makers/component/photostatistic_component.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #108      +/-   ##
==========================================
+ Coverage   30.55%   30.72%   +0.16%     
==========================================
  Files          58       60       +2     
  Lines        3701     3811     +110     
==========================================
+ Hits         1131     1171      +40     
- Misses       2570     2640      +70     

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[tibaldo]

Thank you for all these improvements!