Convert scan_time and time_increment from nanoseconds to seconds for ROS msgs

[rhaschke]

Fixes #31.
ROS msgs expect time_increment and scan_time in seconds (see msg documentation). However, internally scan_time is computed from stamp measures in nanoseconds:

ydlidar/sdk/src/CYdLidar.cpp

Line 136 in ceca2ce

double scan_time = tim_scan_end - tim_scan_start;

ydlidar/sdk/src/CYdLidar.cpp

Lines 83 to 84 in ceca2ce

	tim_scan_start = nodes[0].stamp;
	tim_scan_end = nodes[0].stamp;

ydlidar/sdk/src/ydlidar_driver.cpp

Line 689 in ceca2ce

(*node).stamp = m_ns + package_Sample_Index*m_pointTime;

ydlidar/sdk/src/ydlidar_driver.cpp

Line 683 in ceca2ce

m_ns = getTime() - (nowPackageNum*3 +10)*trans_delay - (nowPackageNum -1)*m_pointTime;