Validation doesn't know about non-required dimensions

[takluyver]

NXDL has a way to record optional dimensions:

<dimensions rank="dataRank">
    <dim index="1" value="nP" />
    <dim index="2" value="i" />
    <dim index="3" value="j" />
    <dim index="4" value="k" required="false"/>
</dimensions>

nxcheck currently doesn't know about this, so if the optional dimension isn't present, it records a warning like

The field rank is 3, but the dimension index of "k" = 4

I think the fix for this is a bit trickier than #247, because the required field is not currently preserved by xml_to_dict(). But I might try to tackle it next week.