Introduce dimsd in all operators

[cako]

Motivation

Many operators change the shape/size of the output. This becomes very hard to keep track of unless the user is very comfortable using that specific operator. Here we propose introducing a dimsd attribute to all operators which take dims. Whereas dims refers to the shape of the "model" array (input to forward) prior to flattening, dimsd would refer to the shape of "data" array (input to adjoint). This way, the user could always rely on the following code pattern for an operator Op acting on some dimensions of an N-dimensional array:

y = (Op @ x.ravel()).reshape(Op.dimsd)
x = (Op.H @ y.ravel()).reshape(Op.dims)

Definition of done

• All operators which have dims should also have dimsd.
• Retire old ad-hoc names like dims_fft in operators which used other attributes to rely on this functionality.

[cako]

Closed by #347