Add plotting support for hexagonal periodic billiard about dynamicalbilliards.jl HOT 2 CLOSED

I think I know of a way to do this:
The distance between opposite walls in a hexagon is sqrt(3)*R with R the outer radius. This is the same number used in the definition in billiard_polygon(6, R, ...).

Make a keyword argument to function plot_billiard(...; ..., ishexagon=false, hexR = 0). If it is a hexagon, (and an herxR is also given) change the plotting replication algorithm:

Instead of replacing by dx, the replicas are created in the all the triangular Bravais lattice vectors (of size sqrt(3)*hexR) until they do not exceed the rectangle defined by xmin, xmax, ymin, ymax, with these x , y being the orbit max and minima.

from dynamicalbilliards.jl.

Finally done on eee7322

from dynamicalbilliards.jl.