Show heterogeneity metrics for evidence synthesis

[schuemie]

An important diagnostic when performing evidence synthesis is the amount of heterogeneity between databases. Both the es_cm_diagnostics_summary and es_sccs_diagnostics_summary tables have an tau and i2 field that express heterogeneity in Bayesian and non-Bayesian meta-analysis, respectively.

Currently there is no place in the Shiny app where one can view these metrics. They might be added to the Diagnostics tab.