improves : core.graph_models 	1. makes time_tree and goals private in Job class 	2. implements methods 		get, set of time_tree 		all, add, remove of goals 	3. removes timewise vaildity for saving a Job instance 	4. implements methods in class Goal 		all, add, remove of parents, children, jobs
improves : core.graph_models
	1. makes time_tree and goals private in Job class
	2. implements methods
		get, set of time_tree
		all, add, remove of goals
	3. removes timewise vaildity for saving a Job instance
	4. implements methods in class Goal
		all, add, remove of parents, children, jobs

Yashasvi-Sriram/Escape-Errands,Yashasvi-Sriram/Escape-Errands,Yashasvi-Sriram/Escape-Errands,Yashasvi-Sriram/EscapeErrands,Yashasvi-Sriram/EscapeErrands,Yashasvi-Sriram/EscapeErrands