Group RGI interface and implementation functions separately.

[tanaya-mankad]

Instead of grouping functions in the implementation file in a publicA-implA, publicB-implB sequence, group all functions from the same class together. This allows us the flexibility of adding implementations that might be used in multiple public interfaces, for example. Secondary grouping is to match the order functions are declared in their respective header files (public first, etc.)

This PR also addresses a minor change in the code checking grid-axis for sortedness, to maintain a comparison operator supporting strict weak ordering for the std::is_sorted function.

[codecov-commenter]

Codecov Report

Merging #33 (42fd4ac) into main (9bab8ed) will increase coverage by 3.32%.
Report is 72 commits behind head on main.
The diff coverage is 85.88%.

@@            Coverage Diff             @@
##             main      #33      +/-   ##
==========================================
+ Coverage   82.56%   85.88%   +3.32%     
==========================================
  Files           5        7       +2     
  Lines         453      517      +64     
==========================================
+ Hits          374      444      +70     
+ Misses         79       73       -6     

Flag	Coverage Δ
integration	85.88% <85.88%> (?)

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

Files Changed	Coverage Δ
include/btwxt/grid-axis.h	0.00% <0.00%> (ø)
include/btwxt/grid-point-data.h	0.00% <0.00%> (ø)
include/btwxt/logging.h	0.00% <0.00%> (ø)
src/regular-grid-interpolator-implementation.h	4.55% <4.55%> (ø)
src/grid-axis.cpp	85.33% <85.33%> (-8.00%)	⬇️
src/regular-grid-interpolator-implementation.cpp	91.11% <91.11%> (ø)
src/regular-grid-interpolator.cpp	97.87% <97.87%> (+11.13%)	⬆️

[nealkruis]

That's pretty phenomenal coverage with those tests!

[tanaya-mankad]

Huh, that's interesting about the tests. Did that happen as a result of refactoring the impl class into its own file?
I have reverted the is_sorted change until we have a chance to understand it better. I'll post it as an Issue so we don't forget about it but for now I think we ought to keep this PR clean with just reorganization.