An option would be to accept a string to set the `interquartile_scale` and allow to pass a float.
You would get something like

```
interquartile_scale: float or string in  ["normal" (default), ],
       The interquartile range is scaled by this factor. If ``interquartile_scale``
       is equal to "normal", the ``interquartile_scale`` will be 
       set such that the data scaled to unit variance. In order
       to converge to unit variance a sufficient number of samples is needed.

```

More choice of automatic setting of interquartile range could be provided.

>It might also be misleading if users really do expect to have an std of 1 after scaling: even with data drawn from a proper Gaussian you need ~1k-10k datapoints until the robust_scale'd data reaches an std of 1.00 (= up to two significant digits).

This kind of remark is very handy in the user documentation guide.