Feature request: include meetings and unscheduled calls signals in flex_index()

[Staceyun]

Is your feature request related to a problem? Please describe.
Meetings and unscheduled calls, apart from emails and IMs, are critical components to understand user collaboration behaviors and are suggested to be added to the flexibility index analysis.

Describe the solution you'd like
Have meetings and unscheduled calls included in the signal extraction. For example, enable users to write, signals = c("email", "IM", "Meetings", "Unscheduled_calls") and extract signals accordingly.

[martinctc]

Thanks @Staceyun! Meetings and unscheduled calls were initially omitted because of the high likelihood of false positives (e.g. meetings accepted but not attended) compared to actual emails/IMs sent. However, we want to provide options for users so we will be implementing this - targeting for patch version v1.6.2.