Function to generate sum-of-variables formula

[nikhilwoodruff]

Would be of the format list[str] -> function[entity, period, parameters -> float]

And then invoked via:

class variable(Variable):
  ...

  formula = sum_of_variables([var1, var2])

[MaxGhenis]

Nice idea. Two questions:

1. Would all variables have to be at the same entity as the variable, or could it do a mix of add and aggr behind the scenes?
2. Could the set of variables come from a parameter?

[nikhilwoodruff]

I think:

1. add will enable either entity level.
2. Nice idea, will implement