Consider reading ion name in <species> tag from id rather than ion in `src/acc.rs` [minor]

[llandsmeer]

The NeuroML2 xsd has the following definition for the <species> tag. Here it lists that we should be reading id rather than ion.

    <xs:complexType name="Species">

        <xs:complexContent>
            <xs:extension base="ValueAcrossSegOrSegGroup">

                <xs:attribute name="id" type="NmlId" use="required"/>

                <xs:attribute name="concentrationModel" type="NmlId" use="required"/>

                <xs:attribute name="ion" type="NmlId" use="optional">
                    <xs:annotation>
                        <xs:documentation>Specifying the ion here again is redundant, the ion name should be the same as id. Kept for now
                            until LEMS implementation can select by id. TODO: remove.
                        </xs:documentation>
                    </xs:annotation>
                </xs:attribute>

                <xs:attribute name="initialConcentration" type="Nml2Quantity_concentration" use="required"/>
                <xs:attribute name="initialExtConcentration" type="Nml2Quantity_concentration" use="required"/>

            </xs:extension>
        </xs:complexContent>

    </xs:complexType>