Adding support for isGenerator tasks

- Changing how DAG is represented, both in code and how DAGs are defined
  in JSON.
- Removing std::vector<Task> representation in favour of a map that will
  enforce unique task names
- Task names now have a name (generated), and a definedName.
- Adding support to loggers to add tasks after a DAGRun has been
  initialized.

TODO.md

@@ -9,6 +9,11 @@ Tasks
         - Allow `{{,!DATES}}` style interpolation, where values from `{{DATES}}` are preserved as a single, comma
           separated list
         - Allow for tasks to define next tasks
+            - Refactor [de]serialization so that a task can be parsed by itself
+            - Add notation of parameterValues
+            - Tasks are now refered by two names:
+                - baseName is the original name in the spec
+                - name is the individual tasks
         - Add execution gates
         - Executors
             - [ ] Slurm Executor