Compute metrics for all data once

Currently the correlations for All data are computed multiple times: first for all data in run_training_analyses and then during the analysis for each subgroup under compute_correlations_by_group.

For various reasons it would be helpful to keep the all_data on top of the subgroup analyses, but we should remove the computation from *_by_group (https://github.com/EducationalTestingService/rsmtool/blob/07f91264795dc8fb5de9518cb65ff4e1685836ef/rsmtool/analyzer.py#L833) and simply concatenate the output of _by_group with all_data computed previously.

Comienza en rsmtool/analyzer.py alrededor de la línea 833 y luego sigue run_training_analyses y compute_correlations_by_group para entender dónde se producen las correlaciones de todos los datos. Confirma que el análisis de subgrupos ya no las recalcula y que su salida se concatena con los resultados all_data calculados previamente, manteniendo all_data en la parte superior.

Escrito por el modelo de indexación a partir del texto del issue.