Re-implement value validation using value classes

[happy5214]

HED 3 introduced the concept of value classes to represent the sets of legal values for each value-taking tag. This will make value validation more robust and schema-driven, as well as allow library schemas to define their own value classes. In order to support value validation using value classes, the value classes have to be parsed (an initial implementation was added as part of the rewrite in #56), and a new validation function must be written to use the value class data required for a given tag. HED 2 will continue to use the existing implementation.