Add EA and EI values to element creation

[ajossorioarana]

Add EA and EI values to element creation, so results of the code in the "Getting Started" page match the images showned (Shear, Bending and Displacement diagrams)

[github-actions]

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[brookshsmith]

Thanks so much for this, @ajossorioarana ! Good pick up. The documentation is only updated each time we release a new version - and I won't release a whole new anastruct version just for this - but once a few more updates get in, then I'll release and your correction will show up on readthedocs.