more accurate area and length calculation

[tyrasd]

makes formulas partially account for the non-spherical shape of the earth (but keeps using spherical approximation for the calculations).

resulting relative errors are now consistently below 0.1% for "small" features (as typically found in OSM). for comparison: before, areas were sometimes off by a few percent, and lengths typically below half a %

also moves a few implementation functions from public to private

Changes proposed in this pull request:

Type of change

• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist

• My code follows the code-style rules and I have checked on the static analyses and benchmark (if applicable) results
• I have commented my code
• I have written javadoc (required for public methods)
• I have added sufficient unit tests
• I have made corresponding changes to the documentation
• I have updated the CHANGELOG.md

🚧 WIP

• check math
• benchmark?
• additional length tests?

[rtroilo]

I didn't do the math, so I hope this is right, but the code looks good :-) 👍

[joker234]

Code change looks good

[tyrasd]

I think this is ready to be merged. Benchmarks would still be nice, but I think the higher accuracy should be worth it in any case. Also, this only adds a few more trig calls, so the performance should be in the same ballpark as before.