Allow all properties to be "nillable"

[allynt]

WHO: @bnlawrence

In CIM1, an enumeration property could be "nillable" which would append a "NONE" value to the list of choices.

In CIM2, all properties can be nillable (see: https://bitbucket.org/bnlawrence/esdoc-nb/issues/225). Where the value is replaced w/ one of:

• nil:inapplicable - there is no value
• nil:missing - the correct value is not readily available to the sender of this data. Furthermore, a correct value may not exist
• nil:template - the value will be available later
• nil:unknown - the correct value is not known to, and not computable by, the sender of this data. However, a correct value probably exists
• nil:withheld - the value is not divulged

Add code to support this throughout the Q. And deprecate the old code that supported nillable enumerations.

[allynt]

This depends upon CIM2 & esdoc-mp being updated to cope w/ this feature.

[allynt]

All properties if defined as "nillable" include a separate template allowing users to specify whether to ignore the value and provide a reason.