`get_measure()`: overview over implemented parameters as `type`

[dimfalk]

mean	min_	max_		date_min_*	date_max_*
temperature	min_temp	max_temp		date_min_temp (***)	date_max_temp (***)
humidity	min_hum	max_hum		date_min_hum (***)	date_max_hum (***)
co2 (*)	min_co2 (*)	max_co2 (*)		date_min_co2 (***)	date_max_co2 (***)
pressure	min_pressure	max_pressure		date_min_pressure (***)	date_max_pressure (***)
noise (*)	min_noise (*)	max_noise (*)		date_min_noise (***)	date_max_noise (***)
rain (**)	min_rain (**)	max_rain (**)	sum_rain	date_min_rain (***)	date_max_rain (***)
windstrength	windangle	guststrength	gustangle	date_min_gust (***)	date_max_gust (***)

(*): Does not seem to be available to the public via /getmeasure since this is essentially private indoor data?

(**): Precipitation values except from "sum_rain" seem to be useless - what's the point of min/mean/max of individual data samples, when you're interested in the integral over time? - especially at higher resolutions. Intentionally ignored to prevent confusion, c.f. #25.

(***): All these timestamps can be derived from the original xts objects and are therefore ignored for now, c.f. #33.