[FEATURE] ArExpression.intVar should allow naming of the variable

When creating a variable using an arithmetic expression, the variable would be more identifiable in output if it could be named for a problem domain concept.

## Expected behavior
ArExpression.intVar accepts an optional `name` argument, that is used instead of the generated name.

## Actual behavior
Currently ArExpression.intVar implementations generate a name based on the arithmetic operator being used, which makes it hard to trace in output.

## Context
When using a model with multiple arithmetic constraints using the same arithmetic operator, it is unclear in any output which variable was generated by which constraint.