small fixes to echograms

[valentina-s]

• added inverted yaxis by default. The hv bug (holoviz/holoviews#5727) got addressed and the plot seems to work without extra axis modifications (fixes #136)
• added a few installation instructions for running pyvista on ec2 ubuntu (for rendering)

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[leewujung]

Great! We really need to have the right side up :)