Start working on CUL-specific mapping

Started by mapping the Cul MODS Digital Origin element from the
MODS element set in Omeka.
Add more mappings from Cul Mods to MODS
Add mapping for Cul MODS Place of Origin
Add DC to mapping functions that map DC fiels in Omeka

So, for example, _mapTitle() is now _mapDCTitle()
Add mapping of the Cul MODS Key Date field.

Added a class for the MODS dateCreated element.
Add mapping of Omeka item ID
set keyDate attribute for MODS dateCreated

When mapping the CUl MODS Key Date field, make set the keyDate
for the MOSD dateCreated element which is the start date, i.e.
either the only date, if there is only one date in the field,
or the first date if the field contains a date range.
Add MODS element recordInfo

This element is required according to
DLF/Aquifer Summary of MODS Requirements and Recommendations Table,
so an function to create one was added, though it is currently
a no-op. Will add default functionality next, when I add subelements
to recordInfo
Add creation of MODS element recordInfo