Release v0.3.0

• added @abc.abstractmethod decorator where appropriate
• added stopped flag to Session to indicate that the execution is over, which is monitored by the seppl.io.execute(...) function
• added wai_logging>=0.0.5 as dependency
• added constants for meta-data types: METADATA_TYPE_STRING, METADATA_TYPE_BOOL, METADATA_TYPE_NUMERIC
• added load_args and save_args methods for loading args from/saving args to files
• the execute(...) method now queries the reader whether it has finished after the first read/yield to allow for dynamically locating files during first call to read() method after initializing the reader
• the placeholders() function (package: seppl.placeholders) now returns the non-input-based ones when outputting input-based ones
• added DataCollector writer, which just collects all the data and makes it accessible
• introduced StreamFilter filters to allow for 1-to-m processing (with m>=0)
• the MultiFilter is now a StreamFilter
• added filter_data generator function and FilterPipelineIterator iterator class for efficiently process data
• the classes_to_str and get_class_name methods can remove builtins. via the clean=True parameter now
• introduced the InifiniteReader mixin which automatically disables batch mode
• added support for caching plugins managed by the ClassRegistry via the ClassCache class (one for each class hierarchy)
• the execute method now supports custom pre-initialization and post-finalization method hooks
• filters output the object ID before processing the incoming data when logging level is set to DEBUG