Can't use TrustRegion with nalgebra types due to Default trait bound

[cfunky]

Hello, I'm currently trying to use the TrustRegion optimizer with the Steihaug subproblem solver and nalgebra's DVector<f64> as underlying ArgminOp::Param type. Unfortunately, this doesn't seem to be possible, as both the Solver impl for TrustRegion and the Steihaug impl require the Default trait to be implemented on the ArgminOp::Param type, which is not the case for the dynamically sized nalgebra types. This is very inconvenient. Interestingly, the only place Default is actually used is to initialize the Steihaug struct which then, as far as I can tell, never uses the initialized default value. A potential fix would be to replace the affected struct fields by Option<ArgminOp::Param> and remove the Default trait bound. What are your thoughts on this?

[stefan-k]

Apologies for the inconvenience and thanks for digging into this. I'm not a huge fan of unwrapping Options but in this case I think it is the only sensible option. I'm always happy to remove unnecessary trait bounds. Would you be willing to provide a PR to fix this? If not I will fix this in the next couple of days.

[cfunky]

Thanks for the quick response. I've created the pull request and referenced this issue.

[stefan-k]

Thanks again! :) Fixed by #192.