Format change for XML in preparation for HED-3G (issue #155)

[VisLab]

We are changing the format for the HED XML schema in preparation for developing XSD to validate.
For reference. Here is the finalized version of the new HED XML format. Starting with HED8.0.0-alpha.2, the HED XML files will have the format listed below. Tools will still read XML files in the old format but will not be able to do full validation on them. Schema developers will be able to read the old format and write the new format to validate.

<?xml version="1.0" ?>
<HED library="test" version="0.0.1">
<prologue>unique optional text blob</prologue>
<schema>
<node>
   <name>xxx</name>
   <description>yyy</description>
   <attribute> . . . </attribute>
   <unitClass>...</unitClass>
</node>
</schema>
<unitClassDefinitions>
   <unitClassDefinition> … <unitClassDefinition>
</unitClassDefinitions>
<unitModifierDefinitions>
   <unitModifierDefinition> … <unitModifierDefinition>
</unitModifierDefinitions>
<schemaAttributeDefinitions> 
   <schemaAttributeDefinition> … <schemaAttributeDefinition>
</schemaAttributeDefinitions>
<epilogue>unique optional text blob</epilogue>
</HED>

Additional details can be found in Appendix B of the specification.