Center assemblies

The code below allows me to stack 2 assemblies but I don't know how to center them, so that they have the same center in the XY plane.

```
rows = 5
cols = 5
pitch = 8

dx = cols*pitch
dy = rows*pitch
dz = 3*pitch
roic = cq.Workplane('XY').box(dx,dy,dz, centered=True)

r = pitch/2/2
h = (2*r)*2
bumps = cq.Workplane('XY').rarray(pitch,pitch,cols,rows, center=True).cylinder(h,r)

assy = cq.Assembly()
assy.add(roic, name='roic', color=cq.Color('pink'))
assy.add(bumps, name='bumps', color=cq.Color('gray'))
assy.constrain("roic@faces@>Z", "bumps@faces@