add glossary of terms

[jkingslake]

as suggested by @gordind, add a glossary:

And ultimately there might be a benefit to having a meta list of key glaciology terms and important equations in a glossary that people could use as a reference for when they're applying what you're teaching them, as you introduce a lot of math for them to work with and having it all in one place to go back to and pull from, as opposed to having to go through the whole textbook to find it, would be nice.