Update to the multi-load situation

With the new approach, we can have the following situation:

                <loadData>
                    <equivLoad/>
                    <contributeLoad id="'1 '" areaNumber="1" zoneNumber="1" offLine="false" name="Load:'1 '(6)">
                        <desc>PSSE Load '1 ' at Bus 6</desc>
                        <ownerList id="1">
                            <ownership unit="PU" value="1.0"/>
                        </ownerList>
                        <constPLoad unit="MVA" re="90.0" im="30.0"/>
                    </contributeLoad>
                </loadData>

We need to update the mapLoadData() method.