Make struct mutable and other improvements

[stecrotti]

Changes:

• Struct is now mutable. Shouldn't be a big peformance issue since typical use does not involve creating many instances in performance-sensitive parts of the code
• Added recompute_partials!(x::UniformIsing) to update the pre-computed quantities stored in the struct after a change in the parameters (J,h,beta)
• Some house-keeping, re-organized files, replaced imports with usings as per the guidelines
• Add a benchmark
• Add some tests

[codecov]

Codecov Report

Attention: Patch coverage is 99.31507% with 1 line in your changes missing coverage. Please review.

Project coverage is 99.51%. Comparing base (ffcb95d) to head (5c5b856).
Report is 18 commits behind head on main.

Files with missing lines	Patch %	Lines
src/uniform_ising.jl	99.26%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main       #7       +/-   ##
===========================================
+ Coverage   84.94%   99.51%   +14.56%     
===========================================
  Files           2        3        +1     
  Lines         186      205       +19     
===========================================
+ Hits          158      204       +46     
+ Misses         28        1       -27     

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