Implementation of a common `TRestRawSignalEventProcess` for rawlib

[jgalan]

Perhaps it would be interesting to define a TRestRawSignalEventProcess where input/output TRestRawSignalEvent types are encapsulated (similar to TRestAxionEventProcess), and thus the implementation of a TRestRawSignalEventProcess gets simplified.

On top of that we could add common metadata members for all rawlib processes that inherit from TRestRawSignalEventProcess such as fSignalRange.

Just an idea to be considered.