Upgrade global planner

[mukundbala]

Fixes issue #7.

1. All common GlobalPlanner utility functions and implementations are now in a common base class GridPlannerCore. Grid based planners like Djikstra, ThetaStar, AStar all inherit from GridPlannerCore
2. GridPlannerCore has pure interface methods (virtual plan(.) , virtual post_process_path(.) and virtual generatePath(.)) functions that all children class must define.
3. OpenList is now implemented as a min heap with 3 different comparator functions. The comaparator functions sort either by g_cost only , f_cost only, or f_cost then tiebreak with g_cost.
4. GlobalPlanner loads in string values of chosen planner name and cost functions and polymorphically selects the appropriate planner using GridPlannerCore base-class pointer to a child class. Virtual method generatePath() is called by Global Planner