fix CondContext.back_prop
> Please write a PR description.

+1  
@albertz  the fix looks good to me. Just needs a clear description. I don't really know what kind of description you want here. I extended it.
Fixes `CondContext.back_prop`. It was obviously wrong before. The old code was:
```python
  @property
  def back_prop(self):
    if self.GetWhileContext():
      self.GetWhileContext().back_prop
    return False
```
So the access to `self.GetWhileContext().back_prop` would not have any effect here, but it is obvious that it was supposed to return this value.
So I changed it to:
```python
  @property
  def back_prop(self):
    if self.GetWhileContext():
      return self.GetWhileContext().back_prop
    return False
```
