Remove DeprecationWarning in simple_linear_interpolation

`simple_linear_interpolation` is rising a `DeprecationWarning`:

> /usr/local/lib/python2.7/dist-packages/matplotlib-1.4.x-py2.7-linux-i686.egg/matplotlib
> /cbook.py:1717: DeprecationWarning: non-integer slice parameter. In a future numpy 
> release, this will raise an error.
>  result[i::steps] = delta * i + a0

This PR is a simple fix to that, which is annoying when drawing polar plots (notably, it bloats firefox when doing this through ipython notebook)