feature: add sustainable web design model

[drydenwilliams]

PR for issue #56

#56

### Acceptance criteria:

• Link to where KWH_PER_GB figure comes from in code comment.
• Fix all GLOABL_INTENSITY typos
• Link to underlying source of carbon intensity info figure of 475, in being explicit about the year in use in code comment
• replace 0.52 magic number with constant explaining what it represents i.e END_USER_DEVICE_ENERGY
• fix annualSegmentEngergy typo
• fix order of variables in energy per visit method, and reformat for more readability as described

[mrchrisadams]

Thanks for this @drydenwilliams.

I've added a few comments and requests for changes - mainly code comments and a typo TBH. If you make those changes I'm happy to merge these in, as it's a definite improvement on what we have right now 👍

[mrchrisadams]

Here's the issue for other bits that came up that I think are worth capturing, but not trying to address in this PR. this would make sense to use in a future version.

#60

[mrchrisadams]

Hi Dryden, thanks for making this - there's a few small changes to make.

[drydenwilliams]

Thanks @mrchrisadams will take a look through this hopefully this week, thanks for taking the time again!

[mrchrisadams]

Thanks for this Dryden!

I'll merge it in now, and make a bit of time to update the README file as outlined in #60, and make any further tweaks needed to get it ready for publishing a new release.