capture and decode raw-data

[avaldebe]

raw sample data

The setup to capture raw data needed for #2 is a bit too involved for casual users.
For example, to capture SPS30 messages (47 bytes long) from /dev/ttyUSB0 one needs to

stty -F /dev/ttyUSB0 min 0 -icanon
tail -f /dev/ttyUSB0 | xxd -g1 -c47

The obscurity of the commands involved is not going to help gaining contributions for #2.
Besides the commands are slightly different for different sensors.

With the introduction of -n N, --samples N on #4, one can further extend the application to provide the raw data on the right format. The following would capture 10 messages, with a frequency of 15 seconds:

pms -s /dev/ttyUSB0 -m SPS30 -n 10 -i 15 raw

capture

Saving raw data to a file could be a good way to create sample data for testing.
This would provide a path to test the command line interface without real hardware attached.

pms -s /dev/ttyUSB0 -m SPS30 -n 10 -i 15 raw --capture

would write the raw data into a file with a well defined name pattern, e.g. start-time.pms. In the previous example, the file name would be something line 2020-01-01_15:22:23.pms

decode

Once we can capture raw data, caould decode it as

pms -s 2020-01-01_15:22:23.pms -m SPS30 -n 10 -i 15 raw --decode

The extension .pms would indicate that we are not reading from a serial port.
The start time is retrieved from the filename. The complete file would be decode if number of samples (-n) is not specified.

combining sub-commands

pms serial and pms csv have some overlap with pms raw-data. Maybe this 3 sub-commands can be rolled into one.
In addition to reducing the number of options, combining this sub-commands would help with the coverage on travis builds.