Backend specification and generic test

[mhier]

Write specifications for the implementation of backends, so that correct behaviour also in "special" situations like exceptions and TransferGroups is guaranteed. From the specifications a series of generic tests should be derived, which can be applied easily to all backends. A start for such generic/unified tests has been made already, see "tests/include/UnifiedBackendTest.h".

The following parts need to be covered (list not complete, please extend):

• backend type registration
• backend instantiation
• register catalogue
• device open and close
• accessor creation
• accessor operation (read, write, all types)
• exception handling shall be included in each part