Restructure scaling in fitness computation

The current implementation is not robust. sclae_fitness can be a bool or a string, yet without checking for non-valid values. minmax scaling is using the first and last value, instead of min and max value?

Additionally, a common scaling approach should be investigated.

The issue names no files, tests, or entry points; begin by locating sclae_fitness and its callers, then inspect how fitness values are scaled. Check the accepted bool and string values and whether minmax scaling uses the actual minimum and maximum. Done means invalid values are handled, scaling behavior is verified, and any additional common scaling approach is resolved.

Written by the indexing model from the issue text.