Sb/step2 refactor process

[basaks]

1. remove unittest suite, and use pytest exclusively
2. prepifg outputs readonly multilooked files.
3. process treats multilooked files as readonly
4. write refpixels to disc and reuse

[codecov-commenter]

Codecov Report

Merging #282 into develop will decrease coverage by 6.05%. The diff coverage is 88.75%.

@@             Coverage Diff             @@
##           develop     #282      +/-   ##
===========================================
- Coverage    92.74%   86.68%   -6.06%     
===========================================
  Files           26       26              
  Lines         3239     3305      +66     
  Branches       505      514       +9     
===========================================
- Hits          3004     2865     -139     
- Misses         149      346     +197     
- Partials        86       94       +8     

Impacted Files	Coverage Δ
pyrate/prepifg.py	53.17% <50.00%> (-41.99%)	:arrow_down:
pyrate/core/orbital.py	88.31% <79.51%> (-7.39%)	:arrow_down:
pyrate/core/timeseries.py	88.34% <80.64%> (-2.17%)	:arrow_down:
pyrate/core/refpixel.py	86.79% <84.09%> (-0.93%)	:arrow_down:
pyrate/core/ref_phs_est.py	88.80% <88.09%> (-1.31%)	:arrow_down:
pyrate/configuration.py	97.91% <100.00%> (+0.18%)	:arrow_up:
pyrate/core/algorithm.py	91.01% <100.00%> (ø)
pyrate/core/aps.py	85.62% <100.00%> (-0.09%)	:arrow_down:
pyrate/core/config.py	93.81% <100.00%> (+0.09%)	:arrow_up:
pyrate/core/covariance.py	96.42% <100.00%> (+0.81%)	:arrow_up:
... and 10 more

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[basaks]

See stack outputs are the same before and after this change:

See timeseries outputs are the same before and after this change:

These are generated using in both develop and the current branch:

pyrate workflow -f tests/test_data/small_test/conf/pyrate_gamma_test.conf