validate nxdlTypes.xsd

[prjemian]

As noted in #1497 (comment), there are 33 errors reported when validating nxdlTypes.xsd.

Character content other than whitespace is not allowed because the content type is 'element-only'.

Fails for each unit example element, like this one:

definitions/nxdlTypes.xsd

Line 93 in c122513

<xs:element name="example">rad</xs:element>

These changes will involve changes to the Python software that creates documentation from this file.