Accounting [Jobs]: don't we need the wallclock time?

Right now we have `ExecTime` as `wall clock time * number of allocated cores`. Once this information is stored, it looks like we have no way to get back to `wall clock time` and `number of allocated cores`.

Out of curiosity, shouldn't we register `WallClockTime` and `NumberOfProcessors` as 2 separate values (that can then be derived to obtain the `ExecTime`).
We could potentially get statistics about the time to execute jobs and the number of processors they are using.

Also the term `Wall clock time` and its definition are widely adopted across batch systems I think, so it could potentially be more intuitive for the people that need to create reports from Accounting DB data for instance.

https://github.com/DIRACGrid/DIRAC/blob/09fdfae3fea92ad8be7614c4c562cc232d8bbbac/src/DIRAC/AccountingSystem/Client/Types/Job.py#L24-L26