In ObservableProperty.clean(), parse_observable() will handle the issue
of non-cyber observable objects being presnt in an observable property.
The line raising a ValueError would not be reached so it was removed.
if no properties are passed to _check_at_least_one_property assume its all properties
when extensions are involved always check them in _check_at_least_one_property (caused by raising the exception on the enclosing type)
added _check_object_constaints to extensions, where appropriate
Added three new exceptions: DependentPropertiestError, AtLeastOnePropertyError, MutuallyExclusivePropertiesError
Added tests for NetworkTraffic, Process, URL, WindowsRegistryKey and X509Certificate
Added error tests for EmailMessage, NetworkTraffic, Artifact,
Added interproperty checker methods to the base class: _check_mutually_exclusive_properties, _check_at_least_one_property and _check_properties_dependency
Added interproperty checkers to Artifact, EmailMIMEComponent, EmailMessage, NetworkTraffic
Made NetworkTraffic.protocols required
Added X509V3ExtenstionsType class
Use EmbeddedObjectProperty for X509Certificate.x509_v3_extensions
uncomment test_file_example_encryption_error
added _check_object_constrains and properties_populated to base class
added ObjectConstraintError
added _check_object_constrains for File
added some test cases
in Observable constructor permit no _valid_refs in kwargs
in Observable._check_property ensure that the prop_name is in the kwargs
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