Spectator observer and convert existing observers to dedicated crates

[stefan-k]

A simple egui observer which shows the progress of the optimization in plots. Plots can selectively be shown or hidden.
This is still very basic and will change substantially until it is ready for merging.

Feedback is welcome!

Todos

• Turn this into separate crate argmin-observer-egui
• Put SlogLogger in dedicated crate argmin-observer-slog
• Remove unnecessary slog_logger feature from argmin
• Change all examples in argmin to use new slog logger crate
• Rename WriteToFile to ParamWriter
• Put ParamWriter in dedicated crate argmin-observer-paramwriter
• Adapt argmin-observer-slog documentation
• Adapt argmin-observer-paramwriter documentation
• Docs on how to install spectator
• Clean up spectator code

Caveats

• Apparently GUIs need to run in the main thread, which is not possible in this case. I solved this by running the GUI in a separate process using the procspawn crate. Samples are then sent via ipc_channel which unfortunately doesn't work on Windows. Therefore Windows support is not possible until there is a reasonable way to communicate with the process that also works on Windows.
• I haven't found a way to redraw the GUI only when new data comes in. Either it is redrawn as often as possible (with every call to update) or only when there is "GUI input" such as mouse movements. I'd like to redraw whenever there is new data on the channel.
• This slows down the optimization substantially, which is probably fine and shouldn't be noticeable for cost functions that aren't super fast to compute. I guess such observers are used only during development/tuning and not "in production", where speed matters.

Changes outside of the new observer

• get_float of KVType also works for all other numeric types now by converting those to floats. int and uint are cast to f64, therefore this operation may be lossy.
• SlogLogger is now part of the crate argmin-observer-slog
• WriteToFile was renamed to ParamWriter and is now part of the crate argmin-observer-paramwriter. The enum WriteToFileSerializer was renamed to ParamWriterFormat and the variant Bincode was renamed to Binary.
• ParamWriter now chooses the extension based on the format. .json for JSON and .bin for Binary.
• The Observe trait now has a observer_final method which is executed at the end of an optimization run.
• observe_init in the Observe trait now has access to the optimization state

Other things for the future

• sloglogger needs tests
• paramwriter needs tests

[codecov-commenter]

Codecov Report

Attention: 654 lines in your changes are missing coverage. Please review.

Comparison is base (220e0d5) 93.30% compared to head (31ccefc) 90.25%.

Files	Patch %	Lines
tools/spectator/src/app.rs	0.00%	286 Missing ⚠️
tools/spectator/src/connection.rs	0.00%	143 Missing ⚠️
observers/spectator/src/observer.rs	56.76%	99 Missing ⚠️
tools/spectator/src/data.rs	0.00%	64 Missing ⚠️
tools/spectator/src/main.rs	0.00%	30 Missing ⚠️
tools/spectator/src/telemetry.rs	0.00%	20 Missing ⚠️
tools/spectator/src/message.rs	0.00%	8 Missing ⚠️
argmin/src/core/kv.rs	91.42%	3 Missing ⚠️
observers/paramwriter/src/lib.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #311      +/-   ##
==========================================
- Coverage   93.30%   90.25%   -3.05%     
==========================================
  Files         117      126       +9     
  Lines       18875    19679     +804     
==========================================
+ Hits        17611    17761     +150     
- Misses       1264     1918     +654     

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