Amend code convention: include headers should use the complete path from the project root

We are mostly, but not completely, consistent with project includes in the code. For the most part we have the full path, but in some files we use relative file locations.

I propose we clean these up and add the following line to the CTA coding conventions:

• CTA code includes must use the full path to the file from the project root. Do not use relative file locations.

Some examples of what I mean below:

// scheduler/OStoreDB/OStoreDB.cpp
...
#include "Scheduler.hpp"
#include "scheduler/LogicalLibrary.hpp"
#include "scheduler/OStoreDB/OStoreDB.hpp"
...

// tapeserver/readtp/ReadtpCmd.cpp
#include "castor/tape/tapeserver/file/FileReaderFactory.hpp"
#include "catalogue/Catalogue.hpp"
#include "catalogue/CatalogueFactory.hpp"
...