v4.0.0

Re-wrote signature checking to support positional only and keyword only arguments.
Deprecated functions get_interface_property_names and get_interface_properties_and_attribute_names have been removed.
PureInterface class has been renamed to Interface to remove the alliteration. PureInterface class still exists but is deprecated.
Concrete class has been removed as it turned out to be unnecessary. Just put object AFTER the interface class.
Fixed cases where properties were being evaluated during class construction.