Weight normalization

A list of weight variables can be specified using the weights-to-normalize and weights-not-normalized entries. A product these variables will be used as weight, with its name chosen according to the following rule: if only one variable is specified, its name will be used as the name of the weight variable; otherwise, weight-var-name needs to be specified to give name to the product variable.

The product is done in the following way:

1. The variables specified in weights-to-normalize are multiplied and the resulting combined weight is normalized to the total number of entries.
2. The variables in weights-not-normalized are then multiplied to the previously normalized weight variable, and the result is the final weight.