Added and example to Envelopes notebook

[AlexGriffiths526]

Hi Colin, sorry it has taken me so long I have had a few problems with my git authentication. Nevertheless, I have put together an example for CS 454 in the Envelopes notebook.

There area couple of points to note:

• To simplify the problem I have limited the application of load to either loading a full span or not at all. This may produce action effect slightly less than the maximum. I think a more elegant method may be to use the coordinates from the IL diagram to position the load where it has the most onerous effect. Like a reservoir filling up if you like.
• The example makes use of the envelope feature but not the load patterning so it might not be exactly what you are after. if it is not what you are after I won't be offended at all if you don't want to include the example.

Thanks again for sharing the library.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.60%. Comparing base (e580334) to head (2cf4a09).

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #55   +/-   ##
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  Coverage   80.60%   80.60%           
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  Files          13       13           
  Lines        1356     1356           
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  Hits         1093     1093           
  Misses        263      263           

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[ccaprani]

Thanks @alexandergriffiths526 ! That's great. I made a few tweaks to make it more generic. It was a really good solution to use the LoadPattern object. However, I realised it could be simpler (at least conceptually, the code is similar) to use the Envelopes class. It just needed a factor_LM method. I'll add this and then revise the notebook accordingly shortly.

Sorry it took so long to get to this, but I'm really grateful for the PR and contribution to the package!