improve readability of mods_display code

[ndushay]

Two points to start:

• how does Field @values get populated?
• what are the objects that are @values

Some of the many uses of “values” in mods_display

• sometimes arrays of Nokogiri XML Elements,
• sometimes strings, and
• Field has @values
• ModsDisplayValues has @values
• And local variables “values” or "value" often used.