Add TCP server

[lluchs]

A new option --csv-server allows receiving the measurement data from another system, for example:

# on the host with the Powenetics device
powenetics-v2 --csv-server '[::]:1234' /dev/ttyACM0

# on the client
nc powenetics-host 1234

Additionally, a --average option enables downsampling to reduce the amount of data transferred or written to file.