Should nan_flag be able to be something else than True/False?

In the current implementation, the ``nan_flag`` parameter has to be either True or False.

I have seen previous implementations where flags are set to nan if the inputs are nan. Currently, this cannot be achieved due to the existing

```python
solarpy.quality.bsrn_limits_flag(np.nan, 45, 1200, nan_flag='a', limits='ppl-ghi')
```