Fix linspace for use with physical quantities

#4336 breaks linspace for use with the quantities package (https://pypi.python.org/pypi/quantities) and perhaps other packages that work in a similar way. 

The specific issue occurs when trying to create a range of numbers with physical quantities, e.g. to get 50 values from one second to ten seconds:

    import quantities as pq
    linspace(1 * pq.s, 10 * pq.s)

will raise an exception because a physical quantity (seconds) and a dimensionless quantity (the 0.0 used in the code) are being added. Since this only occurs when using numpy with additional packages, I haven't opened a new issue for it. But I propose to multiply with one instead of adding zero to avoid issues when using physical quantities.