Investigate optimal tolerances for sweep operations

While investigating #407 I noticed that `BRepOffsetAPI_MakePipeShell` (used by `sweep`) has by default rather loose tolerances. It _seemed_ that making them tighter resulted in more robust bool ops with the generated shape. We might want to change the defaults.

Note to self: here is what I used
```python
builder = BRepOffsetAPI_MakePipeShell(p.wrapped)
builder.SetTolerance(1e-9,1e-9,1e-6)
```