Update heat equation docs

[xtalax]

fixes #86

[codecov]

Codecov Report

Merging #88 (855edcd) into master (54ce54f) will decrease coverage by 7.66%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #88      +/-   ##
==========================================
- Coverage   88.07%   80.40%   -7.67%     
==========================================
  Files          13       13              
  Lines         939      939              
==========================================
- Hits          827      755      -72     
- Misses        112      184      +72     

Impacted Files	Coverage Δ
src/discretization/differential_discretizer.jl	75.67% <0.00%> (-18.25%)	⬇️
src/bcs/generate_bc_eqs.jl	64.89% <0.00%> (-15.96%)	⬇️
...discretization/generate_finite_difference_rules.jl	83.33% <0.00%> (-11.91%)	⬇️
src/MOL_utils.jl	75.39% <0.00%> (-9.53%)	⬇️
src/discretization/discretize_vars.jl	80.24% <0.00%> (-6.18%)	⬇️
src/discretization/MOL_discretization.jl	82.45% <0.00%> (-1.76%)	⬇️
src/bcs/boundary_types.jl	96.90% <0.00%> (-1.04%)	⬇️

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