New gem to parse Library of Congress MODS data

[ronaldtse]

As announced by @kmiller621 here:

• New Schema for allrecords.xml usnistgov/NIST-Tech-Pubs#42

The NIST-Tech-Pubs repository will now be published in MARC21 XML and also MODS.

The MODS schema works easier because it doesn't require us to parse MARC codes which are numerous and prone to errors (from my perspective).

While there are existing gems, it is cleaner for our purpose to implement from a clean slate:

• https://github.com/sul-dlss/mods/blob/master/lib/mods/date.rb

[ronaldtse]

This gem is now released but it will need some aligning to make sure it works with Relaton-NIST.

Ping @andrew2net .