EHN: add error for get_line_intensity

For get_line_intensity, add a ``ValueError`` when the border of the integral is outside the energy range.
Isn't `_get_xray_lines_in_spectral_range` a better name?

Also, woulnd't this method be more useful if it was taking a list of elements as arguments and returning the xray lines in the spectral way. Maybe, if this gets implemented, the method could be public.