Simplified calc_target_index function in stanley_controller.py

[Gjacquenot]

Used:

• np.hypot to replace np.sqrt(x**2 + y**2)
• np.argmin to find location of a minimum in a numpy array

[coveralls]

Pull Request Test Coverage Report for Build 1102

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.07%) to 90.356%

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Files with Coverage Reduction	New Missed Lines	%
PathPlanning/ClosedLoopRRTStar/pure_pursuit.py	2	94.57%
PathPlanning/ClosedLoopRRTStar/closed_loop_rrt_star_car.py	7	68.22%

Totals
Change from base Build 1101:	-0.07%
Covered Lines:	6718
Relevant Lines:	7435

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💛 - Coveralls

[AtsushiSakai]

@Gjacquenot Looks better. Thank you!!