Split test_gamma_ray_grid into two new files in transport folder

[nolanbrown01]

:pencil: Description

Type: :roller_coaster: infrastructure

Split test_gamma_ray_grid.py into test_calculate_distances_3d.py and test_r_packet_transport_3d.py. Moved the tests associated with each file into the appropriate test file.

:vertical_traffic_light: Testing

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• [ ] Other method (describe)
• [ ] My changes can't be tested (explain why)

:ballot_box_with_check: Checklist

• [x] I requested two reviewers for this pull request
• [ ] I updated the documentation according to my changes
• [ ] I built the documentation by applying the build_docs label

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[codecov[bot]]

Codecov Report

Merging #2079 (d76d984) into master (a57ab47) will increase coverage by 0.01%. The diff coverage is 52.94%.

@@            Coverage Diff             @@
##           master    #2079      +/-   ##
==========================================
+ Coverage   58.04%   58.06%   +0.01%     
==========================================
  Files          76       77       +1     
  Lines        8743     8747       +4     
==========================================
+ Hits         5075     5079       +4     
  Misses       3668     3668              

Impacted Files	Coverage Δ
tardis/transport/r_packet_transport_3d.py	42.85% <42.85%> (ø)
...ardis/transport/geometry/calculate_distances_3d.py	19.51% <100.00%> (ø)

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[andrewfullard]

There are duplicated changes between this branch and #2078

[sonachitchyan]

I feel like the name packet for the object might be confusing in the future if we use a different procedure for r_packet and v_packet?

[andrewfullard]

This is outdated