Start of removing hardcoding of length/with values

I would like to use this workbench, but in its current hard coded non-metric values state it is unusable.

I started on attempting to fix this with step 1 being no functional changes, just move constants into one single file which then is imported in files needing those constants.

The first commits were straight forward, but then when I started on replacing hard coded values like 38.1mm with board_width my confidence is relative low since I am not sure that every single instance of 38.1mm in fact refers to board_width. Could you review this commit and confirm or reject?

Does 406.4mm refer to board_centers or stud_centers?