Rename Line's flip()/flip_ip() functions

[itzpr3d4t0r]

I propose renaming the current Line.flip() and Line.flip_ip() methods to flip_endpoints() and flip_endpoints_ip(), respectively. The current names are confusing as they could imply that the entire line is flipped(along x or y axis), when in fact only the endpoints are being swapped. This naming convention are in conflict with potential future functions that flip the line along the x or y axes.

The new names better describe the functionality of the methods and will prevent confusion when using them in conjunction with other line flipping methods that may be added in the future.