Digitizer Block: Rename "poll_rate" to "poll_duration" or change the used unit

[alexxcons]

poll_rate sounds like a frequency (Hz), but we actually work with seconds.

So either rename the variable to e.g. poll_delay/poll_duration (better names welcome!) or change the used unit to Hz.

In any case, a fix in all used flowgraphs will be requird after this change !

To be fixed in the xml of all Digitizer Blocks and in the C++ classes of these Blocks (class "digitizer_block").

[RalphSteinhagen]

My two-pence worth:
I'd recommend using `polling_period' since its a unit of time that is used repetitively.
| <- period -> | <- period -> | <- period -> ...

Using 'delay' suggests IMHO that we wait until something is supposed to start or continue.
| delay -> | start doing something

[alexxcons]

Yep, polling_period sounds good to me !