ENH: Add retrieval and queries to Extern C interface

doc/capi/index.rst.in

include/caosdb/entity.h

include/caosdb/status_code.h

include/ccaosdb.h

src/caosdb/entity.cpp

src/caosdb/transaction.cpp

src/ccaosdb.cpp

test/test_ccaosdb.cpp

test/test_entity.cpp

test/test_transaction.cpp