Add a simple OnSample observer

[ihedvall]

Instead of having the channel observer that collect all samples in the memory, the idea is to only save the last value and have a callback for each sample.

[ihedvall]

The OnSample is now available. Note that it cannot handle VLSD CG i.e. variant length data stored in a CG group. This is sometimes common when doing CAN bus logging (CSS Electronics).