improved antenna pattern load and display. added examples

[ssarris]

@harrison-caudill Note: No modifications were made to link budget calculations that result in the Eb/N0 margin. This PR is all about importing antenna pattern data, display plots, selecting gain value, then doing the same calculations we had before.

Added support for importing antenna pattern data.

• Example in examples/sdown_gs_antpat.py shows import of non-uniform-spaced measurement data from ARA sband dish antenna.
• Example in examples/xdown_sat_antpat.py shows import of Xband patch 3D simulation results and selecting a phi cut to reduce to 2D for use in link budget 2D geometry.
• Greatly simplified the data representation and plotting of antenna pattern data to remove all strange rotation angles and shifts that were present before. Fixed plot of 0 dBi to show negative and positive gridlines and circle at zero rather than a single point.
• Removed unused functions. Many of the unused functions were related to the previously over complicated manipulation of pattern data.
• Added red diamond annotation on TX and RX antenna pattern plots to indicate the gain value that is used in the geometry of the link budget. A little bit of parameter juggling had to happen to make Antenna.plot_pattern() add annotation of selected gain value from calculated link geometry rather than plotting a static pattern from fixed file.

Example output from the recent changes. XD1.pdf SD1.pdf

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This change is 

[harrison-caudill]

@ssarris I was looking through the diff here, and I don't see any substantive changes to the antenna.py or budget.py. It looks like this is primarily about adding the examples and changing apoapsis -> apogee and periapsis -> perigee.

Mind helping me muddle through the changes?

[harrison-caudill]

Closing in deference to #13