Make more clear example of BufferManager

[Nicogene]

We should extend the examples about the BufferManager and make them more clear(e.g. less modern c++ black magic)

[traversaro]

A step in both solving this issue and solving #39 would be to move the configuration of the BufferManager from being done in the constructor, to be done via appropriate configuration methods.

[pattacini]

It's not black magic 😏
It's more about making the example clearer.

Replacing

yarp::telemetry::BufferManager<double> bm_v({ {"one",{4,1}}, {"two",{4,1}} }, 3);

with

// configure telemetry to log 3 samples of 2 variables of vector type whose size is 4 double each
// the channels are called "one" and "two"
size_t n_samples = 3;
std::vector<BufferInfo> channels{ {"one", {4,1}}, {"two", {4,1}} };
yarp::telemetry::BufferManager<double> bm_v(channels, n_samples);

enhances readability.

In my opinion, it's important to introduce the concept of channel, which is not the variable per se to be logged but rather refers to the infrastructure employed to transfer the information.