Defining variables: add simple "alias" pass-throughs

[benkrikler]

Imported from gitlab issue 5

Often it might be helpful to effectively "rename" a variable or just directly use a variable without any calculation (eg you just want to apply a reduction over each event). In that case it could be better for performance and memory use to have the concept of an "alias" where we simply map from one branch name to another.

[kreczko]

Unless I am mistaken, this feature has been available for a while.
In addition, all definitions of the form od_area: {formula: pulsesODHG.pulseArea_phd} should be zero-copy anyway (TBC).