Dimensions of properties like the estimated coefficients in docu (R)

[PhilippBach]

This issue might be relevant for R as well:

https://github.com/DoubleML/doubleml-for-py/issues/85

We should ...

• [ ] ... shortly explain the dimensions all_coef (etc.) in the documentation
• [ ] ... add column and row names to the dimensions of the object

In R we should think of the (currently private) get__...-methods in this context, for example:

https://github.com/DoubleML/doubleml-for-r/blob/8790659a9490c874491fd7de8cc9360b36fcb41b/R/double_ml.R#L784

[MalteKurz]

Moved the naming of dimensions to a separate issue #62