Make airtime a public property and split trackAirtime into calculateAirtime

[amkuipers]

In v1.0, sendBytes updates private airtime in class TheThingsNetwork. In beta, examples just send data and delay for 20s or so. In production there is a need for users to control airtime, and it would be great if the library supports it. I am not aware if TTN Uno has a RTC to 'reset daily used airtime'. But some assistance in calculating a delay/sleep would help. Example: delay_ms = calc_delay( sizeof (user_data_upstream) + sizeof (user_data_downstream )), that uses the private fp and sf, etc, to calculate the minimum delay. If an RTC is available, the used airtime can be recorded per day, and the calc_delay() can use the remaining airtime in the calculation. Documentation then can have examples that use that delay, or less; multiply the delay by 2, or use something like max( calc_delay(..), SIX_HOUR).

[FokkeZB]

I suggest we:

• [ ] Make float airtime public so that users can programmatically retrieve the total airtime to make informed decisions, e.g. to change delays. It also enables them to reset the airtime to 0, e.g. every 24h.
• [ ] Split trackAirtime() to create separate public calculateAirtime() method to allow users to calculate how much a message will cost before sending it.

[johanstokking]

@FokkeZB agreed

[FokkeZB]

Closed by #162