Add work/sleep query option and state tracking variable

function uart_callback(data)

	if sds011.parse_frame(data) then

		-- PM values or work period have been updated

		if sds011.pm2_5i ~= nil then

			-- pm2_5i/pm10i contain the integer part (i.e., PM2.5 / PM10 value in µg/m³)

			-- pm2_5d/pm10d contain the decimal/fractional part (i.e., PM2.5 / PM10 fraction in .1 µg/m³, range 0 .. 9)

		else

			-- sds011.work_period has been updated after using sds011.set_work_period

			-- pm2_5f/pm10f contain the decimal/fractional part (i.e., PM2.5 / PM10 fraction in .1 µg/m³, range 0 .. 9)

		end

	end

end

```

## SDS011 Configuration API

## SDS011 API

If desired, **sds011.lua** can be used to configure the SDS011 sensor.

Currently, the following commands are supported

### Commands

* `port:write(sds011.set_report_mode(active))`

  * active == nil: request current mode; do not change it.

    The mode can be read from `sds011.active_mode` after a few milliseconds.

  * active == true: periodically report PM2.5 and PM10 values via UART

  * active == false: only report PM2.5 and PM10 values when queried

* `port:write(sds011.sleep(sleep))`

  * sleep == true: put sensor into sleep mode.

    The fan is turned off, no further measurements are performed.

  * sleep == false: wake up sensor

* `port:write(sds011.set_work_period(period))`

  * period == nil: request current work period; do not change it.

    The work period can be read from `sds011.work_period` after a few milliseconds.

  * period == 0: continuous operation (about one measurement per second)

  * 0 < *period* ≤ 30: about one measurement every *period* minutes; fan turned off in-between

* `port:write(sds011.sleep(sleep))`

  * sleep == nil: query current sleep mode; do not change it.

    The mode can be read from `sds011.working` after a few milliseconds; do

    not trust its value before that.

    Background: SDS011 sensors only respond to a sleep mode query when they are

    not in sleep mode. To handle this, the driver sets `sds011.working = false`

    when running the query, and reverts it to `sds011.working = true` only if

    it receives an appropriate response.

  * sleep == true: put sensor into sleep mode.

    The fan is turned off, no further measurements are performed. In this mode,

    `port:write(sds011.sleep(false))` is the only command accepted by the

    device.

  * sleep == false: wake up sensor

* `port:write(sds011.query())`: Query PM2.5 and PM10 values in passive mode.

  data is available after a few milliseconds.

### Variables

* `sds011.active_mode`

  * true: the sensor automatically reports readings

  * false: the sensor only reports readings when queried

* `sds011.work_period`

  * 0: perform one reading measurement every second

  * otherwise: number of minutes between measurements

* `sds011.working`

  * true: the sensor is enabled

  * false: the sensor is in sleep mode

* `sds011.pm2_5i`, `sds011.pm2_5f`, `sds011.pm10i`, `sds011.pm10f`: see Usage

## Application Example

local c_work = 0x01

local c_workperiod = 0x08

sds011.work_period = nil

sds011.active_mode = nil

sds011.work_period = nil

sds011.working = nil

function sds011.finish_cmd(cmd)

	cmd = cmd .. string.char(0xff, 0xff)

	elseif active then

		cmd = c_active

	end

	local cmd = string.char(c_head, c_id, c_report_mode, op, cmd)

	cmd = string.char(c_head, c_id, c_report_mode, op, cmd)

	cmd = cmd .. string.char(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

	return sds011.finish_cmd(cmd)

end

function sds011.sleep(sleep)

	local cmd = string.char(c_head, c_id, c_sleepcmd, c_write)

	if sleep then

		cmd = cmd .. string.char(c_sleep)

	else

		cmd = cmd .. string.char(c_work)

function sds011.set_sleep(sleep)

	local op = c_write

	local cmd = c_work

	if sleep == nil then

		-- if the device is sleeping, it will not respond to a query

		sds011.working = false

		op = c_read

	elseif sleep then

		cmd = c_sleep

	end

	cmd = string.char(c_head, c_id, c_sleepcmd, op, cmd)

	cmd = cmd .. string.char(0, 0, 0, 0, 0, 0, 0, 0, 0, 0)

	return sds011.finish_cmd(cmd)

end

	elseif command == 0xc5 and pm25l == 0x02 then

		sds011.active_mode = pm10l == 0

		return true

	elseif command == 0xc5 and pm25l == 0x06 then

		if pm25h == 0 or pm10l == 1 then

			sds011.working = true

		else

			sds011.working = false

		end

		return true

	elseif command == 0xc5 and pm25l == 0x08 then

		sds011.work_period = pm10l

		return true