mechanical.scad: rotational axis not enforced

[jon-gilbert]

(originally opened as https://github.com/jon-gilbert/507common/issues/78)

From the previous redux work on mechanical.scad in https://github.com/jon-gilbert/507common/issues/74 :

This rotation should be a plane perpendicular to the axis (but today is not enforced and should be, could be).

The rotational movement needs to respect the axis Mech attribute: annotations should be made perpendicular to the axis; mechanically aware comparisons and attachments need to consider axis when determining if two shapes "can" with each other.