Add more validation to process

[antho1404]

Current processes still lack a few validations. Right now most of the check is done during the runtime and these checks could be delegated at the creation/compilation.

Some check to add in the Validate function

• [x] Basic struct validation
• [x] Directed tree
• [x] Contains exactly one trigger
• [x] References in map link to an existing node
• [ ] References in map link to an ancestor node only
• [ ] References should map only a task node or a direct parent
• [ ] Type mapping between output/inputs of maps/tasks
• [ ] Filter should filter valid data
• [ ] eventKey or taskKey are defined by the service