Add SPDX license IDs

[bernhold]

I don't know how important/useful this would be, but all the cool kids know about SPDX license identifiers and maybe we should be introducing them into IPS?

Ideas (which need to be vetted):

• In some places I've seen the SPDX id in a separate file
• I've also seen it embedded within the LICENSE
• And here there is a suggestion to use it in individual source files. I'm not clear whether this is a replacement for the full license or in addition (like embedding it into the LICENSE file, above). Should probably check with our IP lawyers as to the desirability of replacement.