[QoL] Add Identity() as default linear in Sparsity prox

[MorganSchmitz]

Is your feature request related to a problem? Please describe. opt.proximity.SparseThreshold requires a linear operator as input I think it'd be good to have a default set up.

Describe the solution you'd like opt.linear.Identity would be the natural choice.

Are you planning to submit a Pull Request?

• [ ] Yes
• [X] No