Create __repr__ and __str__ methods for some classes to give the user a nice overview on printing

[fwitte]

• Units (show current defaults)
• Component (show type, current specifications, result?!)
• Connection (show type, current specifications, result?!)
• Network (?)
• OptimizationProblem (variables, objectives, ...)

[fwitte]

At the community meeting we discussed this topic:

• Components and connections should show a tabular overview of the available specification options and results. The current value of the respective parameters as well as the specification state (user specified or not) should be indicated
• Units will show current defaults