Spectra can now be divided. Error propagation is not yet implemented.
Error propagation implemented when dividing spectra.

The relative errors are added (but the absolute errors are stored).
Merge pull request #51 from ajasja/stderr

Divide Spectra
Implemented Spectrum.y_at_x() which returns a linear interpolated y value at any x between x_min and x_max.

Returns the y value at a specified x value.
        Any values between min and max x are accepted. The returned value is a
        linear interpolation.

        Parameters:
        -----------
            x : array_like
                The x-coordinates of the interpolated values.
        Returns:
        --------
            y : {float, ndarray}
                The interpolated values, same shape as x.
                numpy.NaN is returned for x values outside of xmin and xmax,
Implemented indexing for Spectrum. spectrum[x] now returns a linearly interpolated y-value.
Merge pull request #55 from ajasja/interp

Linear interpolation of y values at given x values