Add absorbance dimension

[nneskildsf]

Absorbance AU is a dimensionless unit, but nevertheless it has meaning beyond just being a number. For instance it is common to convert between AU and mAU.

This is different from, say, the Reynolds Number Re which is proper dimensionless and where a mRe would make no sense.

I propose that absorbance should be added to the default unit registry by defining:

# Absorbance
[absorbance] = []
absorbance_unit = [] = AU

This would be in agreement with how the refractive index unit RIU is handled.

Br Eskild