Optional normalization for relative echo weight

Relative Echo Weight so far automatically normalizes weights echo_intensity / sum(echo_intensities_current_shot)

Let's make the normalization a user-defined option, with a checkbox in the GUI, selected by default and a normalized=true/false parameter in the XML configuration file.

<echo-weight enabled="false" classname="org.amapvox.shot.weight.RelativeEchoWeight">
    <parameters variable="intensity" normalized="true" />
</echo-weight>

For backward compatibility, if normalized parameter is absent then true is assumed.