Function otter_nodejs_tests::api_op_set_z

pub fn api_op_set_z(
    gpc: &mut GPiece, 
    gen: Generation, 
    z: &ZCoord
) -> Result<ShouldSetZLevel, ApiPieceOpError>

Prepare to set the z level of gpc to z

If this is a good idea, returns a ShouldSetZLevel. That should be implemented along with the the rest of the operation, when committing.

This allows us to do all of an operation’s checks and preparation, including Z level setting, first, and then only set the Z level infallibly at the end.