Validate config file

[LukasBommes]

Currently, there are no validation checks performed on the user defined config file. The following checks would be useful:

• validate types of values
• validate ranges of values
• ensure only allowed tasks are in the task list
• ensure order of tasks makes sense
• ensure that tasks that a task depend upon were already run (check if intermediate directories exist in workdir)
• validate that only allowed keys are used