Undid an undesired changed occasionally picked from synchronisation branch

Commit a9158a7 contained not only an improvement in documentation, as it claims, but also an undesired change in functionality. The latter one has been undone manually.
Bugfix: Copy constructor for PECReaderConfig fails when src.bTagDatabase is nullptr

Such situation can happen when user does not need to propagate the b-tagging scale factors and does not provide a database.
A typo in information message has been corrected
Class TriggerRange was moved to dedicated files
Trigger selection was factorised out

The interface for the trigger selection was changed to hide all the access to the trigger tree in the dedicated classes. Now PECReader only gets the trigger tree from a PEC file and deletes it when the file is closed. Events from the tree will be read in a class derieved from TriggerSelectionInterface. This change also prepares to the switch to "slim" trigger results in the new version of PEC files.
TriggerSelectionInterface has been improved

Methods are more logical now: ReadNextEvent is given the ID of the next event in the tree as this information is needed to decide which branches to read. The pointer to the trigger tree has been added as a protected member along with the corresponding event counters; now they are a part of interface. Meaningful constructor are defined. Certain methods turned to non-constant as it suits their nature better. The documentation improved.

Note that this commit introduces compilation errors.
Turn the TriggerSelectionInterface to be non-copy-constructible
Implementation of the trigger selection updated

Now it is aligned with the recent changes in the trigger-selection interface and the switch to SlimTriggerResults plugin in PEC files.
Package aligned with the recent updates in the trigger-selection classes

Also a number of bugs fixed. Now the code can be compiled.
Meaning of the trigger pattern in TriggerRange altered

Now it corresponds to the trigger basename used by the SlimTriggerResults plugin [1] and expected by the rest of the framework's code.
[1] https://github.com/andrey-popov/single-top/blob/master/plugins/SlimTriggerResults.h