Unify asset and ddo, common folder refactoring (#534)

* Unify Asset and DDO classes.
* Move services and delete ddo folder from common.
* Reorganise and remove more stuff from common.

Author: calina-c

READMEs/c2d-flow.md

@@ -164,7 +164,7 @@ provider_url = DataServiceProvider.get_url(ocean.config)
 # returns "http://localhost:8030"
 
 # Calc DATA service compute descriptor
-from ocean_lib.common.ddo.service import Service
+from ocean_lib.services.service import Service
 from ocean_lib.common.agreements.service_types import ServiceTypes
 DATA_compute_service = Service(
     service_endpoint=provider_url,

READMEs/marketplace-flow.md

@@ -155,7 +155,7 @@ service_attributes = {
 # They're only decrypted for datatoken owners upon consume.
 from ocean_lib.data_provider.data_service_provider import DataServiceProvider
 from ocean_lib.common.agreements.service_types import ServiceTypes
-from ocean_lib.common.ddo.service import Service
+from ocean_lib.services.service import Service
 
 service_endpoint = DataServiceProvider.get_url(ocean.config)
 download_service = Service(