Added possibility to use 'default_value' with Variable class

[Endogen]

Description

The class Hash has the parameter default_value which can be used to define a default value for that Hash while initializing it. That same functionality was not available for the class Variable. This PR allows this.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist

• I have performed a self-review of my own code
• I have tested this change in my development environment.
• I have added tests to prove that this change works
• All existing tests pass after this change
• I have added / updated documentation related to this change