raise ValueError when Magnitude of Vector is 0

[arihantparsoya]

Fixes #37.

Added test condition in line 376 to raise ValueError when magnitude of vector is 0.

>>> from p5 import *
>>> x = Vector(0, 0)
>>> print(x.magnitude)
0.0
>>> x.normalize()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/arihantparsoya/Documents/p5/p5/pmath/vector.py", line 368, in normalize
    raise ValueError('Cannot normalize vector with magnitude 0')
ValueError: Cannot normalize vector having magnitude 0

[abhikpal]

Looks good 👍 Have merged into develop