Update landscape plot

[mBarreau]

The current landscape plot is a plot recipe. However, the argument l is already in used for the linewidth size, conflicting thus with the size of the landscape plot. This is not easily possible to draw several times the same landscape so a seed has been added.
Here are the proposed modifications:

• The argument l is changed to lnorm
• There is a random generator introduced if the seed argument is provided.

The tests have been updated and seemingly better looking landscape command has been added to the readme (together with the reference to the original paper proposing this method).

[codecov]

Codecov Report

Patch coverage: 87.50% and project coverage change: +0.15 🎉

Comparison is base (02a8f03) 95.40% compared to head (a7e1d42) 95.55%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   95.40%   95.55%   +0.15%     
==========================================
  Files           2        2              
  Lines          87       90       +3     
==========================================
+ Hits           83       86       +3     
  Misses          4        4              

Flag	Coverage Δ
unittests	95.55% <87.50%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/FluxOptTools.jl	91.11% <87.50%> (+0.63%)	⬆️

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