Add the Scorm2004::Sequencing::Activity class.
Nitpick the comment on Activity#check_sequencing_rules.
Add the SequencingRule#check method.

This method invokes Sequencing Rule Check Subprocess.
Add the Scorm2004::Sequencing::SequencingRulesCheckProcess class.
Add the Scorm2004::Sequencing::SequencingRules module.
Merge branch 'sequencing-rules-check-process'
Add the Scorm2004::Sequencing::Terminate::DescendentAttemptsProcess class.
Merge branch 'terminate_descendent_attempts_process' into sequencing-exit-action-rules-subprocess
Add the Scorm2004::Sequencing::SequencingExitActionRulesSubprocess class.
Add the Scorm2004::Sequencing::ActivityTree class.
Merge branch 'sequencing-exit-action-rules-subprocess'