Added calculated current sensors

[derolli1976]

Description

Since enpal boxes do not provide them anymore, we add calculated sensors for "PowerSensor" Current.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[derolli1976]

Closes #82