Make units of dp 5002 (date) and 99021 (message timestamp) consistent

Pass timestamp as number, not string

Correction to some virtual datapoint units

Allow to retrieve virtual datapoints via request_values call

Also expose frame flags as datapoints of virtual device 'XCOM'

Expose received frame flags:

• is message pending
• was rcc reseted
• is sd-card present
• is sd-card full
• is new datalogger file present

Improve tracing of AC and DC voltages that are used

Support for 24vdc and 12vdc systems

Fix typo: manaul

Fix self._listen_port usage