Appropriately compute "time_until_next_measurement"

[amotl]

People have been asking to compute the appropriate time_until_next_measurement as seen at [1-4] in order to have properly timed measurement intervals.

[1] https://github.com/Hiverize/FiPy/blob/d4c05d12fdf586e69c2296bb1a4ebf300e44f97d/main.py#L75-L81
[2] https://git.cicer.de/autonome-zelle/autonome-zelle-sfwv/blob/9754fb403bf59ef8e5ca4e33f599d20247398407/autonome-zelle-sfwv_v0.0.92.ino#L489-493
[3] https://git.cicer.de/autonome-zelle/autonome-zelle-sfwv/blob/9754fb403bf59ef8e5ca4e33f599d20247398407/autonome-zelle-sfwv_v0.0.92.ino#L507-511
[4] https://github.com/hiveeyes/arduino/blob/0.16.0/node-gprs-http/node-gprs-http.ino#L1095-L1103

[ClemensGruber]

Currently I get with

# Measurement interval in seconds.
'interval': 60.0,

# Whether to use deep sleep between measurement cycles.
'deepsleep': True,

this time stamps:

2019-06-16T19:49:26.048675Z,-0.006
2019-06-16T19:50:00.340812Z,-0.007
2019-06-16T19:50:32.196479Z,-0.008
2019-06-16T19:51:04.122271Z,-0.009
2019-06-16T19:51:35.983008Z,-0.011
2019-06-16T19:52:07.791818Z,-0.008
2019-06-16T19:53:03.541236Z,-0.004
2019-06-16T19:53:35.201671Z,-0.005
2019-06-16T19:54:06.938297Z,-0.006
2019-06-16T19:54:39.002306Z,-0.006
2019-06-16T19:55:10.585772Z,-0.006
2019-06-16T19:55:42.178011Z,-0.008

[amotl]

Thanks, @ClemensGruber. That's the reason for having this issue here, see also
https://github.com/hiveeyes/hiveeyes-micropython-firmware/blob/3844b93e6c088703b9b446e2ce136ff244b8bff4/settings.example.py#L7-L8

Pull requests or suggestions welcome, otherwise this might arrive with one of the next commits.