Cleanup and minor refactoring

CMake clenaup:

• switch to target based approach
• make torch or onnx optional for building
• add standard config flags

ONNXInference clenaup

• Move initialization of input/ouptut names to initialize and do it only once

InferenceInterface

• Make inputs const& to signify to the users that these should not change. Not yet entirely sure about this once, since ONNX requires a const_cast and Torch effectively makes a copy at the moment, so we could in principle also simply take them by value...