DOC: Add an XSD for PSYEXP XML format

The XSD is only basic and does not care about most of the special cases.
For example there is no check for LoopInitiator and LoopTerminator pairs
and if Routines referenced in Flow are actually definied. But structures
and most of the other restrictions (unique names, enumerated valType and
updates, available Components) should be checked.
TEST: Add a test to check demo PSYEXP XML files
DOC: Update because basic XSD now available
DOC: Add missing value types to PSYEXP docs
Merge pull request #630 from philippwiesemann/add-xsd

DOC: Add XSD for PSYEXP XML format and test to check demo files