ScanR: exposure times not always populated

[melissalinkert]

As noted by @mabruce, curated/scanr/embl/Maik_001 has exposure times:

$ grep exposure -A 1 experiment_descriptor.xml 
<Name>AF exposure time [ms]</Name>
<Val>120</Val>
--
<Name>exposure time</Name>
<Val>200.00000</Val>
--
<Name>exposure time</Name>
<Val>500.00000</Val>
--
<Name>exposure time</Name>
<Val>500.00000</Val>

but these are not being populated in the OME-XML (as showinf -nopix -omexml can confirm). It's not immediately clear why that's the case, but pre-dates changes in 7.3.1 so is not a problem that would have been introduced recently.