Add a measurement intervall between readings

[ClemensGruber]

Currently the software starts a new reading after finishing the last round of sensor reading and submitting data. Would be great to add a measurement intervall in the settings file and implement a simple wait periode after last reading.

• intervall in seconds
• start time is before reading sensors so waiting time = sensor reading - data submission
• so we have to check if wait time is negative in case it si very short

[vkuhlen]

I implemented a time.sleep between measurements, the interval can changed in the webinterface in "General settings".