Update copyright headers in kernels.

Update copyright headers in kernel files to match the new format.
Since all the kernel code has been introduced in 2012, removed copyright
years older than that.  The Python script for Verlet kernels was updated
to strip a copyright header from the templates to allow adding one there
as well.  This commit processes all files that are not produced by the
kernel generators, but contain _kernel_ in their path.

Part of #818.

Change-Id: I5d7babc137cdbe0c427cbaa44209b9e8f7be55bb