Add SIG tags to digitizer implementation

[frankosterfeld]

From #28:

While touching the digitizer code ...

... the following might be useful to be modified at the same time: A lot of the functionality contained in 'B1 Scaling & Offset' and our custom 'Cascade' signal sink block should be suitably also integrated into the digitizer block itself, as also proposed to be formalised in GR 4.0 in gnuradio/gnuradio#6253.

Thus the digitizer would generate with its first sample (and/or whenever the information changes) the following tags:

SIG_RATE - the 'uint64_t' encoded sample rate (presently often defined as samp_rate global variable)
SIG_NAME - a 'std::string' encoded name of the signal (N.B. relevant for MIMO & SDRs with a high channel count)
SIG_UNIT - a 'std::string' encoding the physical unit of the signal
SIG_MIN - a 'float' to encode physical DAQ limits
SIG_MAX - a 'float' to encode physical DAQ limits
This reduces the minimum number of required blocks and pools all required source information into the digitizer block. The information of these tags should also simplify the OpenDigitizer functionality listed in fair-acc/opencmw-cpp#197 and provide default values for signal name, sample-rate, etc.