Plot load profiles

[teubert]

Requirement Text
Add the ability to inspect load profiles visually without simulating, when using a standard load class (i.e., part of progpy.loading)

Background Information
Load profiles can be defined using the classes in progpy.loading. However, in order to inspect them, you need to run a simulation.

Suggested Solution
Load.plot(times)

DoD

• Implement feature
• Add to example
• Add tests
• update documentation