Eliminate redundant data processing in mcmc_areas_data()

[utkarshpawade]

Fixes #500

Summary

• Call prepare_mcmc_array(), merge_chains(), and melt_mcmc() once at the top of mcmc_areas_data()
• Pass pre-computed data_long into mcmc_intervals_data() via an internal .data_long parameter

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (042663b) to head (1d6bd1f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #501   +/-   ##
=======================================
  Coverage   98.66%   98.67%           
=======================================
  Files          35       35           
  Lines        5864     5865    +1     
=======================================
+ Hits         5786     5787    +1     
  Misses         78       78           

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