UnitArray __rmul__ and __rdiv__ drop units when isinstance(other, numpy.generic)

I have discovered that scimath.units is dropping the units from the result when the left hand operand to \* or / is a numpy scalar. Simple test cases:

``` python
>>> import numpy
>>> from scimath.units.api import UnitArray
>>> from scimath.units import SI
>>> a = UnitArray(1., units=SI.meter)
>>> b = numpy.array(1.)
>>> a*b # This will work
UnitArray(1.0, units='1.0*m')
>>> b*a # This also works
UnitArray(1.0, units='1.0*m')
>>> c = numpy.sqrt(1.)
>>> type(c)

>>> a*c # this, i.e. __mul__ works
UnitArray(1.0, units='1.0*m')
>>> c*a # __rmul__ fails!
UnitArray(1.0, units='None')
>>> c / a # __rdiv__ shows the same bug
UnitArray(1.0, units='None')

```

It is not immediately clear to me how to fix this.