Explicit License Information

[mpfingsthorn]

Dear all,

the licensing information about your fantastic library/ROS package is not very explicit. The package.xml points to BSD, but it is not clear which one. It would be great if this was made clearer. Ideally:

1. Use a license identifier from https://spdx.org/licenses/ to point to the exact license.
2. Add a LICENSE file as in the kalibr repo: https://github.com/ethz-asl/kalibr/blob/master/LICENSE

We would very much appreciate if you could help us out and clear up these details.

Best regards,
Max