added exercise classes to list of allowed classes for converting to dtbook

fixes #174
use linear="no" for rearnotes, also improve schematron validation

fixes #157

 * added prefixes to all OPF schematron error texts to easily identify the rule in bug reports etc.
 * split all schematron patterns with multiple rules into multiple patterns with only one rule
 * adding linear="no" to rearnotes (dtbook-to-epub3)
Merge pull request #177 from nlbdev/issue-174-157

allow exercise classes (issue 174) and use linear="no" for rearnotes (issue 157)