Introduce a :Subsolver debug symbol and a DebugWhenActive

[kellertuer]

This way adding :Subsolver to a sub solvers debug keyword – the debug is only printed when the parent solver is “active” as well; for now this is mainly corresponding to DebugEvery being on an active iteration.

• unification and documentation of the new set_manopt_parameter! variants
• documentation
• Testing
• a Tutorial, that illustrates this behaviour.

This resolves #189 (since I saw a 1 hour idea how to resolve this finally).

[codecov]

Codecov Report

Merging #285 (8bb6d08) into master (583efd2) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #285   +/-   ##
=======================================
  Coverage   99.73%   99.74%           
=======================================
  Files          73       73           
  Lines        6522     6569   +47     
=======================================
+ Hits         6505     6552   +47     
  Misses         17       17           

Files Changed	Coverage Δ
src/Manopt.jl	87.50% <ø> (ø)
src/solvers/FrankWolfe.jl	100.00% <ø> (ø)
src/plans/debug.jl	100.00% <100.00%> (ø)
src/plans/difference_of_convex_plan.jl	100.00% <100.00%> (ø)
src/plans/objective.jl	100.00% <100.00%> (ø)
src/plans/plan.jl	100.00% <100.00%> (ø)
src/plans/problem.jl	100.00% <100.00%> (ø)
src/plans/subsolver_plan.jl	100.00% <100.00%> (ø)
src/solvers/augmented_Lagrangian_method.jl	100.00% <100.00%> (ø)
src/solvers/difference-of-convex-proximal-point.jl	100.00% <100.00%> (ø)
... and 2 more

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[kellertuer]

I am a bit confused about documenter on CI, since it should run with the current code base where the error that is thrown is actually fixed. Will have to check, why that is happening on the new tutorial.
So locally on my machine that runs fine and it looks like it has Manopt in dev mode so than that error should be fixed but somehow is not. Me very confused.

edit: I think I deconfused myself – and for now not rendering that life and only doing that on a next version should resolve the issue anyways.