Clean API and update documentation

[martin-schlipf]

• Prepare py4vasp for a version 1.0 release by deciding which parts of the API are available.
• Add more documentation for the public part of the API in particular examples.
• Rename classes and methods in the generated documentation, so that they are called Calculation.dos.plot instead of data.Dos.plot
• Write introductory documentation that explains the basics of py4vasp similar to the workshop tutorial
• Change calculations to batch