Add larger size Units K & M

[assignUser]

For velox I am working on tracking binary sizes and have run into bytes being the only available size unit. For the context of build artifacts it's to fine a resolution and with #1380 only half solved with scientific notation it would be great to have an alternative. (I'd prefer M over K if I'd have to make a choice^^). I'd be happy to open a PR to add it to https://github.com/conbench/conbench/blob/d5d9a30682d339f4bc47a96126d2aa230382790f/conbench/units.py which is the only place the would require a change?

While the scientific notation is neat it's un-intuitive for direct comparison if you don't work with it often:

[austin3dickey]

Hey thanks! Yes I believe that file is the only thing you'd need to change, and then you'd need to regenerate the expected docs.

Note that we don't have any protection against the same benchmark being measured in more than one unit, and unfortunately right now if that happens, the plot crashes the page. So after this change, there's a possibility that someone would try to upload results in B and then MB for the same benchmark and be surprised when it fails. That's naturally something we'd want to protect against in the short term and handle gracefully with unit conversion in the long term.