Fix naming conventions in LFMCMC for consistency and clarity

[apulsipher]

This pull request introduces many variable and method name changes to the LFMCMC class. Generally, these changes include:

• Adding the m_ prefix to member variables to improve understanding of variable usage
• Renaming variables to more descriptive names
• Renaming getters and setters to reflect member variable name
• Cleaning up comments and organization of variables and methods

[codecov]

Codecov Report

Attention: Patch coverage is 88.07339% with 13 lines in your changes missing coverage. Please review.

Project coverage is 84.00%. Comparing base (bb8d6e8) to head (b514bd3).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
include/epiworld/math/lfmcmc/lfmcmc-meat.hpp	91.56%	7 Missing ⚠️
include/epiworld/math/lfmcmc/lfmcmc-meat-print.hpp	75.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #37   +/-   ##
=======================================
  Coverage   84.00%   84.00%           
=======================================
  Files          42       42           
  Lines        2769     2770    +1     
=======================================
+ Hits         2326     2327    +1     
  Misses        443      443           

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[apulsipher]

good so far

[apulsipher]

@gvegayon I made the fix to the LFMCMC print statement to include displaying the burn-in period. Also, I updated set_stat_names and set_param_names as you request on the R side

[gvegayon]

Looks good! Just ensure you compile the single header file under ./epiworld.hpp. I think the Makefile has a target for it.