* [SDS011] Add send commands to sensor
In the library used to communicate to the SDS011 sensor was only support to receive data from the sensor and not query the sensor or set another mode.
This is needed for #499 , to let the sensor sleep.
N.B. code is not yet tested, only implemented while reading the datasheet "Laser Dust Sensor Control Protocol V1.3"
* [SDS011] Lessen load on ESP by polling less frequent and use SoftSerial
The default setting were to set the delay to 0 and use an own software serial implementation.
This caused frequent timeouts on the web interface, WiFi reconnects, etc.
Now the already present SoftSerial is being used. This will gather all the data from the serial port and thus there is no need to poll 50 times a second.
Now the ESP is much more responsive.
* [SDS011] Allow the sensor to sleep to extend sensor lifetime
The internal laser of the SDS011 only will last for about 1 year when ran continuously.
When the RX of the sensor is connected, it can be set to sleep for N minutes between measurements. This will extend the lifetime of the sensor and reduce power consumption.
This sleep time is stored inside the flash of the sensor, so it can be set once and then the RX on the sensor could be disconnected again to be used for other purposes.
TD-er
Jochen Krapf