ARW: Get black level and white point from EXIF

[artemist]

This implements the change suggested in #349 so that Sony cameras with white and black level encoded in their metadata do not require sensor rules in cameras.xml

Unfortunately I haven't been able to test on high ISO samples because I cannot find samples at ISO 102400 with a properly set camera model name and I do not currently own an ILCE-7M4

[codecov]

Codecov Report

Base: 59.58% // Head: 59.67% // Increases project coverage by +0.08% 🎉

Coverage data is based on head (2935bba) compared to base (d723728).
Patch coverage: 92.30% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #385      +/-   ##
===========================================
+ Coverage    59.58%   59.67%   +0.08%     
===========================================
  Files          207      207              
  Lines        12625    12638      +13     
  Branches      1766     1770       +4     
===========================================
+ Hits          7523     7542      +19     
+ Misses        4964     4958       -6     
  Partials       138      138              

Flag	Coverage Δ
integration	47.59% <92.30%> (+0.10%)	⬆️
rpu_u	47.59% <92.30%> (+0.10%)	⬆️
unittests	21.79% <0.00%> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/librawspeed/decoders/ArwDecoder.cpp	49.45% <92.30%> (+2.88%)	⬆️
src/utilities/rstest/rstest.cpp	53.33% <0.00%> (+0.83%)	⬆️
src/librawspeed/common/ErrorLog.cpp	64.28% <0.00%> (+21.42%)	⬆️

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[LebedevRI]

@artemist Thank you!