make OMDoc interface logic independent

[till]

first complete task #42!

make OMDoc interface logic independent:

• add the necessary logic-specific conversion functions to Logic.Logic.Logic
  • for each of sign, morphism, sentence, add conversion functions from/to an appropriate datatype in source:trunk/OMDoc/OMDocInterface.hs
  • e.g. for conversion of CASL morphisms to OMDoc: the function is OMDoc.OMDocOutput.createOMMorphism
• move the CASL-specific conversion functions from OMDoc/ to CASL/OMDoc.hs, and the logic instance in CASL.Logic_CASL
• replace signature, morphism and sentence conversion functions in OMDoc with calls to the logic independent functions in Logic.Logic.Logic
• create a table with a correspondence "logics in Hets" - "OMDoc URIs"
• create a table with a correspondence "comorphisms in Hets" - "OMDoc URIs"
• use all this for the translation of structured heterogeneous specifications