Adding correct naming scheme for train config sections

Summary

This MR introduces the following changes

• Renaming NN_structure to nn_structure
• Renaming Validation_metrics_settings to validation_settings
• Renaming Eval_parameters_validation to evaluation_settings
• Renaming LRR* to lrr*
• Renaming DIPS_ppm_units to dips_ppm_units
• Renaming DIPS_ppm_condition to dips_ppm_condition
• Renaming DIPS_dense_units to dips_dense_units
• Renaming DL1_units to dl1_units
• Renaming N_Conditions to n_conditions
• Renaming Batch_Normalisation to batch_normalisation

Relates to the following issues

• One task of #167 (closed)

Conformity

• Changelog entry
• Documentation
• Development guidelines
• Style guides