Fixed small bug in neural network and written tutorial on it

[ludoro]

No description provided.

[codecov]

Codecov Report

Merging #220 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   84.89%   84.90%   +0.01%     
==========================================
  Files          20       20              
  Lines        1734     1736       +2     
==========================================
+ Hits         1472     1474       +2     
  Misses        262      262              

Impacted Files	Coverage Δ
src/NeuralSurrogate.jl	100.00% <100.00%> (ø)

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