change miligram per day to milligram per liter

[cmungall]

Fixes #129

But please check with @pnrobinson what was intended here before merging, I assume that even though mg per day is a valid derived unit it's not what was intended

[pnrobinson]

We should probably choose one way of doing this:

UCUM:mg/dL  
UCUM:mg.kg-1    

[ielis]

I must admit I like the -1 version much more, but can we actually do that?

[cmungall]

There is a separate issue for normalizing on the exponent form #131, this PR is just about fixing a typo