Give RNG a seed for reproducability

[xtalax]

fixes #127

[codecov]

Codecov Report

Merging #128 (f1a5314) into master (a740aef) will increase coverage by 7.55%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #128      +/-   ##
==========================================
+ Coverage   80.74%   88.29%   +7.55%     
==========================================
  Files          13       13              
  Lines         940      940              
==========================================
+ Hits          759      830      +71     
+ Misses        181      110      -71     

Impacted Files	Coverage Δ
src/bcs/boundary_types.jl	97.93% <0.00%> (+1.03%)	⬆️
src/discretization/MOL_discretization.jl	84.21% <0.00%> (+1.75%)	⬆️
src/discretization/discretize_vars.jl	86.41% <0.00%> (+6.17%)	⬆️
src/MOL_utils.jl	84.12% <0.00%> (+8.73%)	⬆️
...discretization/generate_finite_difference_rules.jl	95.29% <0.00%> (+11.76%)	⬆️
src/bcs/generate_bc_eqs.jl	80.85% <0.00%> (+15.95%)	⬆️
src/discretization/differential_discretizer.jl	95.94% <0.00%> (+18.24%)	⬆️

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