ENH: add .dropinf() and/or .dropna(drop_inf=True) methods
There are known ways of droping inf and nan (.replace(np.inf, np.nan).dropna()) but still, the API could be expanded:

```
.dropinf(inplace=False) 

.dropna(drop_inf = False)
```

Are both self explanatory wishes and it is no stretch to see how they are usefull. 
Note for instance that today's API does not provide a one liner to drop inf in place while keeping nans.

Best

PS: not sure how to submit wishes.
