Admit ProximalBase 0.6 in compat when it is released (package, exper, benchmark/compare/new) #17

The problem

ProximalBase master is heading for a breaking 0.6 release: mlakolar/ProximalBase.jl#193 has merged, and #142, #261 and #269 there are planned for it. This package's compat is ProximalBase = "0.5" in Project.toml, exper/Project.toml and benchmark/compare/new/Project.toml. CI tests against the sibling masters (scripts/test_master.jl). So once ProximalBase's Project.toml says 0.6, every CI job here stops at resolution, as happened for ProximalSolvers 0.3 in #15.

Found while fixing #15 (PR #16). CovarianceSelection has the same follow-up: mlakolar/CovarianceSelection.jl#60.

Proposal

• When ProximalBase's version moves to 0.6, set ProximalBase = "0.6" in the three files, in the same PR that adapts to any 0.6 change, after ProximalSolvers and CovarianceSelection have moved.
• Run the full suite against the masters at that commit, and resolve exper/ and benchmark/compare/new/, which CI does not run.
• This package reads the internal fields nzval and nzval2ind of a SymmetricSparseIterate (src/reduced_diff_estim.jl, src/inv_hessian_estimation_sym_reduced.jl). SparseIterate and SymmetricSparseIterate: what nnz counts, and findnz/nonzeros for the symmetric iterate ProximalBase.jl#192 proposes public findnz and nonzeros for it; move to those when they exist.