Add functionality to compute FU (Fatty acid uptake) images similarly to GU

[buccimtpc]

Possibility to join glucose and fatty acid as tracee in a single column to pick values from AIVO2.
Possibility to rename GU and FU parametric images to TU tracee uptake images, or read label from a list, or according to tracer specific naming. If fdg > GU, if FTHA > FU and so on. not so many tracers.

The formula for FU calculation is : FUR x Plasma FFA (the tracee value)

[tkkarjal]

A couple of comments:

1. The glucose and gu options are indeed somewhat problematic as they only relate to FDG-studies. It would make much more sense if it were possible to generalise this somehow. As far as I understand, for FDG the Ki*[peripheral plasma glucose concentration] has a biological interpretation, because Ki refers to the transport rate of glucose into the cells, and presumably the [peripheral plasma glucose concentration] is a good approximation (?) of [central plasma glucose concentration]. The product thus describes how much glucose got trapped into the cells during the time frame. However, as far as I know there is no such direct interpretation for e.g. neuroreceptor studies.

2. Maybe we indeed need to rename the glucose and gu columns to make it more generalisable. Vesa speaks here (http://www.turkupetcentre.net/petanalysis/model_ki.html) of 'analog of a native substrate'. If it's always plasma concentration that we are talking about, then perhaps glucose should be replaced by c_nat. Maybe gu could be replaced by nu, i.e. native (substrate) uptake?