Add a mechanism to create process dependencies

[jgalan]

Sometimes a process will require that a process has been placed previously inside the data chain. Then, it would be interesting to introduce a mechanism to indicate those dependencies inside the process.

We could think of something like inserting a line of code as follows in our process:

// Values 1 or 2 define if the process is to be recommended or required. It could be an enum giving better legibility to the code.
ProcessRequired("TRestMyRecommendedProcess", 1);
ProcessRequired("TRestMyRequiredProcess", 2);

or

ProcessRecommended("TRestMyRecommendedProcess");
ProcessRequired("TRestMyRequiredProcess");

or

AddProcessDependency("TRestMyRecommendedProcess", 1);
AddProcessDependency("TRestMyRequiredProcess", 2);

Then, the processing will warn in case of a recommended process is not present, or prompt an error in case a required process is not available.

@rest-for-physics/core_dev