compendium entries with URLs should use https:// instead of http:// where possible

[dkg]

Several entries in the compendium have a URL that uses http:// when the target site offers (and, these days, often prefers) to use https:// instead.

For example, http://ieeexplore.ieee.org/document/1055638/ should probably be https://ieeexplore.ieee.org/document/1055638/, and http://www.sciencedirect.com/science/article/pii/S1361372397895486 should probably be https://www.sciencedirect.com/science/article/pii/S1361372397895486.

If the bibtex export were in git, i could offer a PR to transform the ones where https:// is an option, but i don't have a good way to sync that back to zotero, so i'm just reporting it as an issue here.