Refactoring of `EkfacInfluence`

[schroedk]

By now, there is the inheritance TorchInfluenceFunctionModel->EkfacInfluence
and EkfacInfluence is the only existing subclass.

In order to reduce code duplication:

• Check, if it makes sense to inherit from TorchComposableInfluence
• If not, integrate functionality from base class TorchInfluenceFunctionModel
  into EkfacInfluence
• Remove abstract base class TorchInfluenceFunctionModel