Weighted L2 norm - weight with a CIL linear operator?

Description

Discussed with @bosschmidt at the CIL user meeting hackathon.

For the weighted L2 norm in CIL, the weight value can be a scalar or can be a CIL data container :

CIL/Wrappers/Python/cil/optimisation/functions/L2NormSquared.py

Line 125 in 5d569dc

    
               weight: a `scalar` or a `DataContainer` with the same shape as the intended domain of this `WeightedL2NormSquared` function

Mathematically, $F(x) = |x|_{W,2}^2 = \Sigma_iw_ix_i^2 = \langle x, Wx\rangle = x^TWx$ where $W=\text{diag}(weight)$ if weight is a DataContainer or $W=\text{weight} I$ if weight is a scalar.

It might be useful to extend this so that $W$ is a linear operator whose range and domain are the same and contain the input $x$.

This would allow the weighted L2norm to be weighted by non-diagonal matrices.