Implemented theta function

[mivanovska007]

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 75.00%. Comparing base (75204b4) to head (f4aa540).

❗ Current head f4aa540 differs from pull request most recent head 5348405

Please upload reports for the commit 5348405 to get more accurate results.

Files	Patch %	Lines
src/cartesian.jl	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #5      +/-   ##
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- Coverage   83.96%   75.00%   -8.97%     
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  Files           3        3              
  Lines         131      108      -23     
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- Hits          110       81      -29     
- Misses         21       27       +6     

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[Moelf]

one thing I realized is that, delta theta is ambiguous, it can mean one of the two things:

1. delta theta like delta eta, which is what we have in this PR -- you take thetas of the two 4-vectors, and you can take the diff between those two thetas
2. delta theta like delta R, which is what we used in our offline discussion, this is the opening angle (not just in theta direction) between two 4-vectors

I think it might be wise to use the second definition instead.