Coordinate conversion inside plotting module not handled correctly

[ritzvik]

🐞 Problem
Earlier, plotting was written with Schwarzschild in mind, which relies on Spherical Coordinates only. But as the plotting was extended for Geodesic object, it inherently deals with Kerr & KerrNewman also. The coordinate conversion inside ScatterGeodesicPlotter and StaticGeodesicPlotter is not taken care by coordinates module and the design assumes Spherical to Cartesian conversion by default(No BL taken in consideration). This leads to wrong calculations.

See https://github.com/einsteinpy/einsteinpy/blob/master/src/einsteinpy/plotting/geodesics/geodesics_scatter.py#L62-L63

🎯 Goal

• Fix this

💡 Possible solutions

• Maybe, use return_cartesian = True in calculate_trajectory() to get values directly in x,y,z form.

📋 Steps to solve the problem

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[ritzvik]

@shreyasbapat @Varunvaruns9 , kind of an urgent problem!

[shreyasbapat]

Oh my my!

This needs a fix!

[shreyasbapat]

Closed in #274