Changes to Py5Vector set_mag() and normalize()

[AsadNizami]

Fix for #243.
set_mag() works when the magnitude is negative. However for cases like (0, 0, 1) it returns (-0, -0, -1). Is this okay, or should I change it to (0, 0, -1)?

[hx2A]

I think it is OK. @villares , what do you think?

In cases like this, we often compare py5 with Processing to get an idea of what is appropriate or what users would expect. Processing's API is a useful guide.

For this Processing code:

PVector v1 = new PVector(0, 0, 5);
PVector v2 = new PVector(0, 0, 0);

println(v1.copy().setMag(-1));
println(v1.copy().div(-1));
println(v2.copy().normalize());

[ -0.0, -0.0, -1.0 ]
[ -0.0, -0.0, -5.0 ]
[ 0.0, 0.0, 0.0 ]

If Processing is OK with negative zero, py5 should be also.

This PR would do the exact same thing as Processing except normalizing a zero vector would issue a warning 'Using normalize on a vector of zero magnitude has no effect'. I would maybe change the wording of that to "zero vector" instead of "vector of zero magnitude", but other than that, I think this PR is great.

[villares]

Wonderful improvement! Thank you @AsadNizami!

[AsadNizami]

Pls review!

[hx2A]

Thank you, @AsadNizami !