Add Line `flip()`

[itzpr3d4t0r]

I propose adding the following new methods to the Line class:

• flip_x() -> flips the line along the x-axis
• flip_y() -> flips the line along the y-axis
• _ip versions

This proposal should follow issue #182, where we rename the following methods:

• Line.flip() -> flip_endpoints()
• Line.flip_ip() -> flip_endpoints_ip()

This should be done to avoid confusion with line flipping along the x and y axis.

Alternatively, we could consider adding a general flip()/flip_ip() method that takes an axis as an argument. This would allow users to flip the line along any arbitrary axis.