Viz tab: Improve verbiage for "parent entity" in mouseovers for non-compatible variables

[danicahelb]

When a variable is not compatible for selection in visualizations (ie, x-axis selector, overlay selector, etc), we provide mouseover text with a brief description of why. The verbiage for the mouseovers often refers to "parent entity", which should not be user-facing language. We need to determine better terminology and update the mouseover text

[danicahelb]

Perhaps we could use "observational unit" as external facing language instead on "entity"? see: https://cran.r-project.org/web/packages/tidyr/vignettes/tidy-data.html

so "Parent observational unit" instead of "Parent entity"?

[danicahelb]

dependent on decisions made in VEuPathDB/EdaNewIssues#287