Fix slope calculation; add alternative methods

[spahrenk]

Description

• These changes address issues with calculating slope used for cubic splines.
• Three methods are available from which the user can select for slope calculation: quadratic, cardinal, and finite-difference
• Tests were expanded to verify expected results in all three cases. The default "quadratic" method is used, unless otherwise specified.
• The markdown file was expanded to inlcude some background theory. Documentation for multi-axis spline was not expanded.

Author Progress Checklist:

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    • exercising the code changes in the test framework, and/or
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Reviewer Checklist:

• Read the pull request description
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• Iterate with author until all changes are approved

[tanaya-mankad]

I think we plan to leave the Readme file as a general overview with basic build and usage instructions, and keep detailed documentation in a separate location, but this content is a great start for docs.
My 2c, if I were a user of btwxt I would prefer a tutorial-style walkthrough of the theory rather than a textbook chapter; i.e. instead of concrete applications being "left as an exercise to the reader," they are instead peppered throughout the derivations, helping users understand the behavior of the library for specific choices, and not only its most generalized mathematical theory.

[codecov-commenter]

Codecov Report

Merging #31 (5045a2e) into main (9bab8ed) will increase coverage by 3.76%.
The diff coverage is 86.30%.

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   82.56%   86.32%   +3.76%     
==========================================
  Files           5        6       +1     
  Lines         453      563     +110     
==========================================
+ Hits          374      486     +112     
+ Misses         79       77       -2     

Flag	Coverage Δ
integration	86.32% <86.30%> (?)

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

Impacted Files	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	13.79% <13.79%> (ø)
src/grid-axis.cpp	87.07% <87.07%> (-6.26%)	⬇️
src/regular-grid-interpolator.cpp	92.70% <92.68%> (+5.96%)	⬆️