Flatten all the list types in STIXType

[sbarnum]

Currently STIXType contains list types (e.g. IndicatorsType) for each top-level construct that are nothing but containers for 1..* instances of that type.

Propose flattening these to simply be of the contained object type with a 0..* multiplicity.

[athiasjerome]

This seems an interesting approach for simplification.
Meantime that would (maybe) also avoid the work of reviewing the (as of now not existing) naming convention for these list types
For now: +1 for me