Added conversions for stamp types

Hi @william. How do you feel about that small change?

It allows for neater code when updating the stamp of a pcl point cloud:

compare

std_msgs::Header h = pcl_conversions::fromPCL(pcd.header);
h.stamp.fromSec( /*some double value here*/ );
pcd.header = pcl_conversions::toPCL(h);

or worst:

pcd.header = /*some double here*/ * 1e6;

with:

pcd.header = pcl_conversions::toPCL(ros::Time(/*some double here*/));


(I do this a lot)