added forceReadings() for force mode

[drewfish]

This adds a forceReadings() method which puts the sensor into force mode. When this happens the BME reads the sensors, stores the results in the data registers, then goes to sleep. Another call to forceReadings() is then needed to cause the sensor to read again.

Using this I saw the device go from using a steady ~400uA to a base of ~0.4uA and spiking to ~50uA. (OK, my multimeter is pretty cheap, as is my EE knowledge 😄) I was measuring 3 times a second with no filtering and 1x sampling of all three readings.

[drewfish]

FWIW, this PR is mainly in case others are interested. I don't intend to use it since the delay() will cause trouble for my event loop.

[PaulRB]

Thanks @drewfish, sounds like this would fix my issue #6. Can you please confirm? If so, I will close my issue.

[drewfish]

Yeah it sounds like it might. To be sure you should try it yourself. You can hopefully find some guides on the net on how to use the github version of an arduino library (this one), and then another guide on how to "checkout a pull request" (this one).

[ripper121]

Is there a Library where this function is already implemented?

[nseidle]

We've added a .setMode(MODE_FORCED) function, .isMeasuring() function, and an example demonstrating how to put the BME280 into low power mode (see example 6) in the latest lib v2.x.

Thank you for this pull request! I'm sorry we couldn't use it in the overhaul but your feedback pushed us to include this feature.

If you get the chance, please kick the tires of the new library version and let us know if you run into any problems.