Add rasterXY serpentine raster path generator

[sina1988]

This PR adds a new rasterXY(...) helper to fullcontrol.geometry.waves.

Why:

• squarewaveXY(...) was not flexible enough for my use case
• I needed a simple serpentine raster over a rectangular XY area
• the final pass is forced onto max_y

Notes:

• alternates direction on each pass
• intended for roughing / raster-style toolpaths

[AndyGlx]

This looks good. Before I merge, what do you think about forcing step-over distance to be equal throughout? At the moment, the last raster is spaced up to 1.999999 times the normal raster spacing in order to fill the space. But it might be nive to distribute that gap across all rasters (and maybe print out the actual raster spacing for the user to see). Then you would have an extra line something like:

stepover_to_fill = (max_y - min_y)/n_passes

and then in the for loop:

y = min_y + i * stepover_to_fill

or this could be added as a toggle for something like auto_stepover with default value False

[sina1988]

Thanks, sounds good. I agree an auto_stepover=False toggle is better.

Small point: since there are n_passes - 1 gaps, I think it should be:

stepover_to_fill = (max_y - min_y) / (n_passes - 1)

[AndyGlx]

Yep definitely. Good maths!