Complete first testing&documentation pass for remaining endpoints

[brookslogan]

We are merging #23 as-is. There are a few infrequently-used endpoints that have some remaining TODOs from this first pass. Quoting from the list there:

• dengue_nowcast (need valid locations, but API is lacking) EDIT: done in first pass endpoints vignette (for closing #96) #102
• dengue_sensors (need valid locations and names, but API is lacking) EDIT: done in first pass endpoints vignette (for closing #96) #102
• ecdc_ili EDIT: done in first pass endpoints vignette (for closing #96) #102
• kcdc_ili (need valid region, but API is lacking) EDIT: done in first pass endpoints vignette (for closing #96) #102
• paho_dengue (bug prevents testing) EDIT: done in first pass endpoints vignette (for closing #96) #102

[dshemetov]

fixed by #102