Add vector addition and subtraction operations.

[dylannalex]

Given the vectors $\vec{v}$ and $\vec{w}$, we want to compute $\vec{n} = \vec{v} + \vec{w}$ and $\vec{m} = \vec{v} - \vec{w}$ as shown below:

import curvipy

v = curvipy.Vector([10, 10])
w = curvipy.Vector([5, -5])
n = v + w
m = v - w

This is not possible on Curvipy 1.0.1. To do so, users have to calculate $\vec{n}$ and $\vec{m}$ manually:

import curvipy

v = curvipy.Vector([10, 10])
w = curvipy.Vector([5, -5])
n = curvipy.Vector([10 + 5, 10 + (-5)])
m = curvipy.Vector([10 - 5, 10 - (-5)])