Cleanup rtperiodicdiffar

[damonbayer]

• Renames internal variables
• Removes RtWeeklyDiffARProcess entirely
• Makes log_rt_init_rv, autoreg_rv, and periodic_diff_sd_rv into non-RandomVariable arguments to RtPeriodicDiffARProcess.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.51%. Comparing base (650bf27) to head (80eb58f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #419      +/-   ##
==========================================
- Coverage   93.53%   93.51%   -0.03%     
==========================================
  Files          41       41              
  Lines        1021     1017       -4     
==========================================
- Hits          955      951       -4     
  Misses         66       66              

Flag	Coverage Δ
unittests	93.51% <100.00%> (-0.03%)	⬇️

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[dylanhmorris]

Could be good to have a test that makes use of both the vectorized differenced process and the vectorized AR process to make sure they work together. Perhaps as a test for rtperiodicdiffar. Could be done here or as part of #419

Originally posted by @damonbayer in #446 (comment)