Merge branch 'f-cpp-to-string' into 'dev'

include/caosdb/data_type.h

include/caosdb/entity.h

include/caosdb/protobuf_helper.h

include/caosdb/transaction.h

include/caosdb/value.h

include/ccaosdb.h

src/caosdb/entity.cpp

src/caosdb/file_transmission/download_request_handler.cpp

src/caosdb/file_transmission/upload_request_handler.cpp

src/caosdb/protobuf_helper.cpp

src/caosdb/transaction.cpp

src/ccaosdb.cpp

test/test_ccaosdb.cpp

test/test_data_type.cpp

test/test_entity.cpp

test/test_list_properties.cpp

test/test_utility.cpp

test/test_value.cpp