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

[PhilippBach]

This issue might be relevant for R as well:

DoubleML/doubleml-for-py#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:

doubleml-for-r/R/double_ml.R

Line 784 in 8790659

get__psi_a = function() self$psi_a[, private$i_rep, private$i_treat],

[MalteKurz]

Moved the naming of dimensions to a separate issue #62