Add standard alignment markers

[emmakbat]

It would be nice to have standard alignment marker placements so that every time we go to do a multilayer process, we know where to expect them to be when we go to the MLA and so on. This could be added at what is currently the "Design" level, and we could have perhaps a standard formula as a function of size (e.g. place at (x_size/2 - 100 um, y_size/2 - 100 um). That way it is flexible yet predictable.

The first-issue version of this task would be to add four alignment marks to the "Design" level in the high-current layer with some formula that makes sense to you, and then get feedback on whether that seems to work well when you do the pull request.

Best alignment marks for us are a cross with a narrow inner cross (in case of aligning another e-beam layer) and then the arms become wide and long (for aligning MLA), but just regular big crosses would be okay for a start too.