a few small tweaks to the public API

[antond-weta]

Description

Small tweaks to the public API to squeeze in before 2.0.
The changes are really minor:

• add a const qualifier to a few methods
• make the reference to hints constant in configure()

Tests

no new code introduced

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable. (Check if there is no
  need to update the documentation, for example if this is a bug fix that
  doesn't change the API.)
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.56%. Comparing base (439d1fb) to head (290f765).

Files with missing lines	Patch %	Lines
src/rawtoaces_util/image_converter.cpp	66.66%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   72.53%   72.56%   +0.02%     
==========================================
  Files          10       10              
  Lines        2112     2114       +2     
  Branches      317      317              
==========================================
+ Hits         1532     1534       +2     
  Misses        580      580              

Files with missing lines	Coverage Δ
include/rawtoaces/image_converter.h	100.00% <ø> (ø)
include/rawtoaces/spectral_data.h	100.00% <ø> (ø)
include/rawtoaces/usage_timer.h	100.00% <ø> (ø)
src/rawtoaces_core/spectral_data.cpp	78.46% <100.00%> (ø)
src/rawtoaces_util/usage_timer.cpp	100.00% <100.00%> (ø)
src/rawtoaces_util/image_converter.cpp	54.74% <66.66%> (+0.08%)	⬆️

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[soswow]

love it. method should be verbs