Move code around for better understandability

[sunxd3]

• No alterations have been made to the code; only documentation has been enhanced and typos corrected.

Change Details:

• The NodeFunctions have been relocated to compiler_pass.jl.
  • This transition aims to consolidate common functions and the CollectVariable pass with NodeFunctions, thereby fostering a deeper understanding of the code associated with NodeFunctions, even though it results in a lengthier file.
• The contents of bugsast.jl have been transferred to parser.jl.
  • The renaming of @bugs made the bugsast title somewhat ambiguous. Moreover, due to the similar functionalities of parser.jl and @bugsast, I think their merging makes sense.
• The sections of code related to the Model have been isolated in a distinct file.
  • This separation promotes a more streamlined abstraction of the Graph. By doing this, the Graph interface can potentially be reused, possibly paving the way for the development of graph-based inference algorithms that are independent of the Model definitions in BUGS.
• Several functions have been migrated to utils.jl to minimize the scattering of code and maintain a cleaner code base.

[codecov]

Codecov Report

Patch coverage: 68.85% and no project coverage change.

Comparison is base (e9f705a) 69.20% compared to head (4e809a5) 69.20%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #85   +/-   ##
=======================================
  Coverage   69.20%   69.20%           
=======================================
  Files          17       17           
  Lines        1640     1640           
=======================================
  Hits         1135     1135           
  Misses        505      505           

Files Changed	Coverage Δ
src/JuliaBUGS.jl	94.87% <ø> (ø)
src/graphs.jl	75.38% <ø> (+4.21%)	⬆️
src/parser.jl	76.27% <49.02%> (-16.42%)	⬇️
src/model.jl	64.18% <64.18%> (ø)
src/utils.jl	90.24% <90.24%> (ø)
src/compiler_pass.jl	91.64% <92.39%> (+0.73%)	⬆️
src/variable_types.jl	78.57% <100.00%> (+2.57%)	⬆️

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[yebai]

Thanks @sunxd3 -- it looks good to me!