Add concept of "type" to LearningStandards

The Ed-Fi model of learning standards seems to be missing a concept in many other standards models: the concept that there is an additional classification of the type of the specific learning standards.

As a example, take the Next Gen Science Standards - each of their standards is classified as a "core idea", "practice" or "concept" - see https://www.nextgenscience.org/sites/default/files/resource/files/How%20to%20Read%20NGSS%20-%20Final%2008.19.13_0.pdf  

This concept also appears in CEDS (see Competency Framework Item Type)  and in the CASE specification (as CFItemType)