Changed result to modified Accessible Area

The area of each polygon cell is divided by distance between target
point and center point of the cell.
Displayed result is the sum of these modified areas.
Merge branch 'master' of https://github.com/SNRLab/PercutaneousApproachAnalysis

Conflicts:
	PercutaneousApproachAnalysis/PercutaneousApproachAnalysis.pyc