Prevent writing to things that shouldn't be written to

Add a list of things that shouldn't be written to and patch up all the ways to access them, like the `__getitem__` function. Goal is this:

```python
with bwpy.File(...) as f:
  f["3BRecInfo/3BRecVars/BitDepth"][0] = 12
>>> RuntimeError: The bit depth is a protected variable. If you wish to venture into dangerous lands open the file in unsafe mode.
```