Add new ReductionVariable constructor with Operation as parameter

[scastd]

Describe the feature

Add a new constructor for the ReductionVariable that takes an Operation and calls the other constructor.

public ReductionVariable(Operation operation, T initialValue);

Additional information

• [X] Would you be willing to help implement this feature?

Final checks

• [X] Read the contribution guide.
• [X] Check existing discussions and issues.