Attributes field in docstrings of operators

[mrava87]

Motivation

Historically, every operator has a docstring containing an Attributes section with 2 fields: shape and explicit. This isn’t very informative and outdated as by now each linear operator has more ‘shared’ attributes (common to all operators) than those two. Moreover different operators have different attributes that may be useful to tell users about.

Definition of done

• Add an Attributes section to LinearOperator which details all of the attributes that are shared among operators (eg shape, dims, dimsd, explicit)
• Remove or modify the Attributes section in every operator. Remove if the operator does not define interesting attributes in the init method, of modify removing shape and explicit and including those if it does.