Structures_Engine: Add create methods for Connection

Create method needed for Connection class - once this is created - depends on initial class from [Tekla.oM](https://github.com/BuroHappoldEngineering/Tekla_Toolkit/issues/6)

at least 2 create methods are needed:

- create from existing library; using already defined standard connections from e.g. Tekla
- create from scratch; manually adding all components of the connection.