DISC algorithm modifies input zenith series

Here's the offending [lines](https://github.com/pvlib/pvlib-python/blob/master/pvlib/clearsky.py#L360):

``` python
    Ztemp=SunZen
    Ztemp[SunZen > 87]=87
```

`Ztemp` references the same object as the input `SunZen`, so the `87` assignment rudely modifies the input object.

This can be fixed by using `Ztemp=SunZen.copy()`.

Can be closed in the same PR as #6.
