Remove neps argument 'searcher_path' and yaml argument searcher_kwargs + New loading design for optimizer

[danrgll]

• remove neps argument searcher_path
• remove neps yaml argument searcher_kwargs
• new loading design for optimizer (searcher="path/to/my/custom_searcher.yaml")
• rename key 'algorithm' to 'strategy' for searcher
• Defining custom_searcher.yaml within run_args is now possible e.g.

  searcher: 
     name: "my_searcher" #optional 
     strategy: bayesian_optimization
     initial_design_size: 7

• Closes Standardize yaml path definitions for the neps arguments #70
• Closes Enable argument modification for custom optimizer class that has been loaded via yaml #69

[eddiebergman]

Looks good as a far as I can see (hard to validate strings everywhere tbh)

One bug that needs fixing, rest is fine!

[eddiebergman]

Seems like the test failures here were fixed by @karibbov in ade69db , many thanks :)