How to tell if Accessor is Registered
I have a simple question:

How do you know if your accessor namespace has already been registered with the extension framework?


Let's say I do:

```python
import pandas as pd
import coolaccessor # registers dataframe accessor/namespace
```

Now later on in the code, a person does:

```python
def cool_function(a,b):
   import coolaccessor
   return pd.cool.shibby(a,b)
```

It will try to re-register the accessor.  This produces a warning during testing if that case occurs.  So how can I check that if on re-import the registration doesn't need to occur again?