refactor `anyUnitsAttr`

[prjemian]

per PR #549 (for #545), consider the change in nxdlTypes.xsd of anyUnitsAttr from:

<xs:simpleType name="anyUnitsAttr">
	<xs:restriction base="xs:string">
	</xs:restriction>
</xs:simpleType>

to:

<xs:simpleType name="anyUnitsAttr">
	<xs:union memberTypes="xs:string">
	</xs:union>
</xs:simpleType>

and then converting <xs:enumeration to <xs:simpleType for each type of units

IMPORTANT The result of these changes in nxdlTypes.xsd should bump the minor version number of the XML Schema.

[prjemian]

testing will require merging with recent commits to master but those merges have some conflicts ... needs more time to investigate ... later

[prjemian]

Since the unit types are enumerated in anyUnitsAttr now, the python code to convert units to rst must be refactored. It would assist that code if the field and group types also defined in nxdlTypes.xsd were enumerated similarly. This new enumeration anyTypeAttr is not currently referenced in validation; it will help documentation and possibly data file validation code.

Alternatively, the units could be moved into a separate file such as nxdlUnits.xsd. That might be clearer.