Minor `pvc_cap()` changes

[jon-gilbert]

Two identified todos for pvc_cap():

• a cap perhaps doesn't need to have the same wall thickness as the pipe it's being attached to.
  Maybe? I'm not sure it matters to the construction of the model, its use with other parts, or functionally in the real world.
• caps probably also have a slightly domed top.
  This is based on what's seen in person at the local hardware store. This is an aesthetic change.