Support mapping of `CurrentTransformer` and `PotentialTransformer` CIM elements

Acc. to the [mapping document](https://github.com/com-pas/compas-cim-mapping/blob/develop/MAPPING.md) this piece of functionality is able to convert CIM `CurrentTransformer` and `PotentialTransformer` to the SCL `ConductingEquipment` of type`CTR` and `VTR`, respectively. 

**Requirement**
- make sure `CurrentTransformer` and `PotentialTransformer` are mapped to SCL


**Testfile**
- tbd