Cache model-point independent variables

[zchmielewska]

If you look at a model, there are variables that are not dependent on the model point.

For example, in the basic term, the model starts with:

@variable()
def duration(t):
    return t//12

@variable()
def inflation_factor(t):
    return (1 + assumption["inflation_rate"])**(t/12)

@variable()
def age(t):
    return main.get("age_at_entry") + duration(t)
    
... 

From these 3 variables, duration and inflation_factor don't differ between model points so we could calculate it only once.

So generally speaking all variables that don't depend on model points data can be calculate only once.