Return values of cim_import

In the current version (8b86df1f) the function cim_import has three return values: res, namespaces and url_reference_dict. res contains all objects created accessible via the mRID. The information stored in namespaces and url_reference_dict are only used for the export function. These three return values should be merged into one dictionary called import_result with this structure:

import_result = {'meta_info': {'author': ..., 'namespaces': [...], 'urls': {...}},
                 'topology': {mRID: obj, mRID: obj, ....}}