Move more packages to weak dependencies

[kellertuer]

As discussed in #656, this is an idea to move two further packages to weak dependencies.

I am not saying we should necessarily do that, but it is something to discuss, and I for now just spent about an hour to reorganise the code. I am not yet sure the advantages outweigh the disadvantages here

This would be a breaking change

• ArrayPartition will no longer be exported in Manifolds.jl
• we currently loose some default implementations on product manifolds and groups (for RecursiveArrays) and a few random functions (for staticArrays) which could also be provided again as default without the package dependency for sure.

Before checking and adopting the tests I would like to discuss whether we actually want to do this, with the disadvantages above the question is, does the reduced loading time make this worth the change?

[mateuszbaran]

I've taken a look at this now and I think removing StaticArrays as a dependency would be too annoying to pull off: too many methods use it internally for speed. RecursiveArrayTools, on the other hand, should be doable without major issues. Another dependency I'd prefer to have as a weak one is Distributions.jl.

I expect to have enough free time in August to push for a breaking release containing transition to weak dependencies + Oliver's PR (with polishing). What do you think?

[kellertuer]

Yes, now that I noticed that Oliviers PR is breaking, we could combine that with this PR and check a bit what else to break since we are at it ;)

Thanks for looking into it.

[kellertuer]

We could for example also do #701 then. That should even be a quite short PR.

[mateuszbaran]

Yes, #701 could also be done, or any other quick and breaking thing we intended to do.

[mateuszbaran]

732 is now merged and supersedes this PR so we can close this.