[eigen] Add new project to *Projects using Eigen*

[madplotlib Cpp <madplotlib@xxxxxxxxx>]

Hi, 

There's a new project out there called Madplotlib that makes it easier to plot 2D charts from data created by Eigen::ArrayXf. 

It does sounds remarkably similar to Matplotlib (Python), doesn't it?! But there's a reason why! Follow this link to see some screenshots:

https://github.com/madplotlib/madplotlib

Can somebody be nice enough to add a reference in Projects using Eigen section of the Wiki?

Thank you!

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Karl Phillip Buhr