Respect PAGE-XML ReadingOrder elements

[memowe]

ReadingOrder elements are optional, ALTO doesn't have them. So if not present, the reading order should be linear.

<ReadingOrder>
    <OrderedGroup id="ro_1777378701884" caption="Regions reading order">
        <RegionRefIndexed index="0" regionRef="tr_1"/>
        <RegionRefIndexed index="1" regionRef="tr_2"/>
        <RegionRefIndexed index="2" regionRef="tr_3"/>
    </OrderedGroup>
</ReadingOrder>

[memowe]

This doesn't seem to appear in our input data from pero-ocr. No reason to not do it though.