Fix property handling in showProjection

[jamesmkrieger]

We have had a number of problems with this, two of which I have attempted to solve in #1070 and #1073. I think the problem is that this function is a bit of a hybrid of plt.scatter and plt.plot with some custom things that don't make a lot of sense and may have got broken easily as a consequence. This may also be due to differences between the 2D and 3D versions of these functions.

It could be useful to look at the blame on this file (https://github.com/prody/ProDy/blame/master/prody/dynamics/plotting.py) and work out what changed when and why.

Here's an early version of the code using plt.plot for 2D and mpl_toolkits.mplot3d.Axes3D.plot for 3D, which very clean and simple using matplotlib to handle everything. Obviously, there are reasons for changes since this point, but it looks like a good model to strive for.
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