Add classical Heisenberg model

[ffreyer]

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TODO:

• [ ] use jackknife (or ErrorPropagator) for cV
• [ ] re-add magnetic susceptibility
• [ ] test results (currently only T << T_C, T_C and T >> T_C are checked, but only T << T_C is confirmed)
• [ ] check if docstrings still make sense

[codecov[bot]]

Codecov Report

Merging #44 into master will decrease coverage by 4.36%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   74.76%   70.39%   -4.37%     
==========================================
  Files          25       27       +2     
  Lines         967     1027      +60     
==========================================
  Hits          723      723              
- Misses        244      304      +60

Impacted Files	Coverage Δ
src/MonteCarlo.jl	100% <ø> (ø)	:arrow_up:
src/models/Heisenberg/HeisenbergModel.jl	0% <0%> (ø)
src/models/Heisenberg/measurements.jl	0% <0%> (ø)
src/lattices/LatPhys.jl	0% <0%> (ø)	:arrow_up:

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