Bug/feature request: convert/convert_unitted

[I feel like I have had this conversation before, but can't find trace of it anymore. Hope it is not a duplicate]

I keep forgetting that the `convert` function only works for float, not `UnitScalar`s. As a result, and because `UnitScalar` is a subclass of `np.ndarray`, the following happens:

```
In [3]: from scimath.units.api import convert, UnitScalar

In [4]: x = UnitScalar(1., units="cm")

In [5]: from scimath.units.length import meter

In [6]: convert(x, from_unit=x.units, to_unit=meter)
Out[6]: UnitScalar(0.01, units='0.01*m')
```

which feels very buggy. I believe that we should:
1. either raise an exception when a `UnitScalar` is passed, and create a new `convert_unitted` function that accepts `UnitScalar`s/`UnitArray`s
2. or support the `UnitScalar`s correctly.

I think I like option 2 better, but it makes things not backward compatible, so is probably a no go.

Opinions? @timdiller @rkern ?