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