Description

Several changes to the clock class. The main ones are:

Replace the increment and decrement clock methods with + and - operators that allow to add and substract ticks to a clock.
Compute granularities on-the-fly when comparing clocks. This way there's no need to store the "global" clock granularity, which in turns allows to simplify the code in several places, as it is not necessary to obtain the smallest algorithmic step before initializing clocks.

News snippet

Several improvements to the clock class.

Checklist

I have checked that my code follows the Octopus coding standards
I have added tests for all the new features added in this request.