Fix warnings

[amoodie]

So I actually checked on #24, and there are only two print warnings in the bmi codes. I just changed them in place to use actual warnings, without changing anything else.

Moving forward, we just want to enforce using warnings rather than just printing info.

Closes #24

[amoodie]

I clearly screwed something up with my branching, because the first two commits are supposed to be part of #37. Merge #37 first, then this PR. Should fix history when doing that.

[elbeejay]

Warnings all work as you'd expect. Can be merged after the pre-requisite PRs have been handled.

[amoodie]

thank @elbeejay I fixed the commit history, git rebase to the rescue! so I'm going to merge.