add function to convert units

[dlebauer]

Initial implementation of a function to perform unit conversion without converting to unit class. Fixes #398

Currently duplicates ud_convert defined in the auto-generated RcppExports.R, not sure where the function should live, but currently passes devtools::check()

[codecov]

Codecov Report

Attention: Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Project coverage is 91.86%. Comparing base (e0988ce) to head (c06e4f4).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
R/udunits.R	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #399      +/-   ##
==========================================
- Coverage   91.89%   91.86%   -0.04%     
==========================================
  Files          19       19              
  Lines        1024     1032       +8     
==========================================
+ Hits          941      948       +7     
- Misses         83       84       +1     

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[Enchufa2]

We cannot just overwrite the RcppExports function. With these changes, I have

• Renamed the C++ function, so that ud_convert can be implemented and exported independently.
• Added support for symbolic_units to ud_convert.
• Simplified ud_are_convertible in a similar way.
• Merged the manual pages and move some tests around to better reflect the code structure.

Any further comments?

[Enchufa2]

Thanks!