Start fleshing out the CalibrationStore interface

[gdetrez]

Some more progress towards #147:

• remove the dictionary interface of CalibrationStore and replace it with more explicit methods
• start moving the responsibility of saving the CCCs to CalibrationStore. Only simple ones for now but I suspect that I'll want/need to do larger refactorings for the other ones so they can be separate PRs.

This is a refactoring, it is not intended to change the behavior at this point.

[codecov-io]

Codecov Report

Merging #313 into master will decrease coverage by 0.11%. The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
- Coverage    36.2%   36.09%   -0.12%     
==========================================
  Files         167      167              
  Lines       16349    16350       +1     
  Branches     2897     2897              
==========================================
- Hits         5919     5901      -18     
- Misses      10007    10029      +22     
+ Partials      423      420       -3

Impacted Files	Coverage Δ
scanomatic/data_processing/calibration.py	69.06% <84.61%> (+2.2%)	:arrow_up:
scanomatic/io/ccc_data.py	67.78% <0%> (-20.14%)	:arrow_down:
scanomatic/data_processing/phases/segmentation.py	74.02% <0%> (+1.11%)	:arrow_up:

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