Ingredient.substance.strength.presentation is a Ratio.

But in some cases there is no denominator available. Up to now this has been done by just leaving it empty but that is not technically legal. The correct way to do this is to use Ratio when there is a ratio and just a Quantity when it is not. 

There are also cases where a coded strength is wanted such as "sufficient quantity". 

The Medication resource solves this by having Strength[X] which is a choice of Ratio|CodeableConcept|Quantity

We should align the main Ingredient resource strengths with that. i.e. 

Ingredient.substance.strength.presentation

Ingredient.substance.strength.concentration

to be Ratio|CodeableConcept|Quantity

The high limits and reference strengths don't need a code, it would not make sense

Ingredient.substance.strength.presentationHighLimit

 Ingredient.substance.strength.concentrarionHighLmit

 Ingredient.substance.strength.referenceStrength.strength

Ingredient.substance.strength.referenceStrength.strengthHighLimit

 to be Ratio|Quantity