Workflow validation

[antho1404]

Add workflow validation:

• Basic validation of the required attributes
• Graph structure validation:
  • Do not allow graph with cycles
  • Do not allow graph with multiple roots, (non-connected graphs)
  • Do not allow graph with nodes that contain more than one parent

With this validation, we are allowing only directed tree graphs which is a family of DAG.

You can use the test from this PR: #1209

[krhubert]

With this PR a lot of workflow methods handle the graph and not workflow itself. As in previous PR I would love to see the part where the graph is a separate struct.

[krhubert]

I've never see a mono-parental word to describe a graph, can you point out the article/algorithm where they use such terminology?

Suggested change

	return fmt.Errorf("non mono-parental graph")
	return fmt.Errorf("the edge %s has to or more parents")

Also, a method name can be changed.

[antho1404]

This is not the right term, I couldn't find a better name. If you know any name that describes that each node should have only one parent I'm really happy to change this

[antho1404]

changed the function to maximumParentNode(max int) that returns true when all the nodes have less than max parents.

[antho1404]

With this PR a lot of workflow methods handle the graph and not workflow itself. As in previous PR I would love to see the part where the graph is a separate struct.

I agree that's why I started to move all the graph-related stuff in the algorithm file. I don't want to charge too much this PR and also I have to have a look at https://github.com/gonum/gonum/tree/master/graph that might replace all. So this will certainly change soon.

[NicolasMahe]

it seems good except the function hasNodes that return the opposite of what the function's name is saying. The name should also by singular hasNode has it should true if at least 1 node exist.