Make Seal5Flow a ContextManager

**Before:**

```python
seal5_flow = Seal5Flow(dest, name=name)
seal5_flow.initialize(...)
...
```

**After:**

```python
with Seal5Flow(dest, name=name) as seal5_flow:
    seal5_flow.initialize(...)
    ...
```

**Advantage:**
- Allows cleaning up the logging server introduced in #190 more easily.