rdtech-um: send analog values in frames not samples

rdtech-um: send analog values in frames not samples

The RDTech UM energy meter provides six analog channels, the values of
which are taken at the same time from one consistent raw data image.
Send these values in frames, so applications can tell they are related.

This drops support for --samples specifications. And allows to read back
previously specified time limits.

Tested by means of:
  $ sigrok-cli -d rdtech-um:conn=bt/rfcomm/$MAC --show
  $ sigrok-cli -d rdtech-um:conn=bt/rfcomm/$MAC --frames 4