Add check for Non-Numeric/Invalid Entries in Aggregate (of Attributes) Functions (sum/avg)

[TobiUUlm]

fixes #80
Sample for testing:

features
    Bike
        optional
            Front {Weight 2}
            Back {Weight 1.5}
            Dynamo {Weight 3, Manufacturer 'sample'}
constraints
    avg(Weight) > 0
    avg(invalidattr) > 0
    avg(Manufacturer) > 0
    sum(Weight) > 0
    sum(invalidattr) > 0
    sum(Manufacturer) > 0

[felixrieg]

LGTM

When using a attribute from an imported uvl file I still get the error message.
But since this is also the behavior on main, maybe this is a separate issue