Handle constants

[jankatins]

We currentlx want to implement some workaround which manifests as a constant factor in a metric: basically "Overhead = [Composed Metric] * factor". Unfortunately this isn't really possible as the parser cannot handle the constant factor in a formula, so I have to either implement the overhead formula in SQL (meaning I have to reimplement the compose metric :-() or add a column (and add it as simple measure) which represents the factor (bad as we will in the next step replace the constant with some dynamic thingy...).

For now I will do the first, but having the ability to specify a constant factor in a composed metric would be awesome...