DMN17 — Allow modellers to define range types

DMN 1.6 supports the semantic domain for ranges (see 10.3.2.7 Ranges and Figure 10-26: FEEL lattice type).

However, the Meta Model & XML schema do not support the definition of such types in an Item Definition.

Add flag isRange to MM & XSD

Allow modellers to define range types by adding an extra attribute, called isRange, similar to the one used for lists: https://github.com/omg-dmn-taskforce/omg-dmn-spec/blob/master/xsd/DMN16.xsd#L245