Added valueClass to schema

[VisLab]

The beta.3 version of the schema and XSD have the following updates:

1. The textClass, nameClass, numericClass, posixPath, and dateTime are now part of a new valueClass not unitClass.
2. The isNumeric schema attribute has been removed.
3. A valueProperty has been added to the schema attribute properties.

This PR partially addresses the following issues:

• XML format change to include property #225
• Determine scope of date-time values #194
• File paths as value tags #174
• Need per-time units for rate and date-time units hed-schemas#73
• Create strict guidelines for characters to be used in new tags as well as labels and descriptions #33
• Distinguish integer-only numeric values #19.

Note: I hand edited both the MEDIAWIKI and the XML so there might be errors. The XML validates with the XSD but was produced manually, rather than by the hedtools.

I think we should go ahead with this modification unless serious issues are raised. It gives us a path forward to address the issues listed above in a consistent way. If we adopt, the tools need to be able to read schema in the new form, but taking advantage for validation does not need to be included in this release. It can be addressed in the future.

Please comment @IanCa @happy5214 @dungscout96 @sappelhoff