Dimpled Tube

Hi Guys,
I'm really interested in seeing if I can leverage this tool to create a variety of tube geometries for a heat exchanger investigation.

I've gotten as far as building a tube and adding spheres, but I'm having trouble figuring out how to cut off their tops, and cut out a slightly smaller sphere.

Tube Creation
```python
import cadquery as cq

height = 0.005
width = 0.025
length = 1
thickness = 0.0001
fillet = (height/2)*0.99
fillet1 = ((height-2*thickness)/2)*0.99
external = cq.Workplane("XY").box(height, width, length).edges("|Z").fillet(fillet)
internal = cq.Workplane("XY").box(height-2*thickness, width-2*thickness, length*1.1).edges("|Z").fillet(fillet1)
external = external.cut(internal)
```

Followed by sphere creation
```python
s = cq.Workplane("YZ").workplane(offset=0.0025).rarray(0.005, 0.01, 3, 50, True).sphere(0.001)
```
I had a lot of trouble getting spheres to work on an rarray, but the above seems to work fine.

And now to cutting the tops off.. this doesn't seem to work with spheres...

```python
result = s.faces(">Z").workplane(-0.0001).split(keepTop=True)
```

The end goal is something like this:

Can someone give a couple hints on how to cut out the top of the spheres, and hollow them out?