Iso Var: Manage variables with multiple values

This class can manage variables with multiple values.

It can create and manage a variable assigned to a given context and perform several types of operations with one or more values assigned to the variable. Currently it can:

- Add multiple values to a variable
- Get the variable values are all of the same type,
- Iterate over the values of the variable
- Get the maximum, minumum or sum of the variable values
- Get a list of unique values of the variable
- Call a given function for each of the variable values
- Link a variable to another