Fix registry instances

[s-kania]

This is my draft proposal to fix #8.
What I changed?

• instead of creating a new instance of a node that is then added to the registry and used in multiple trees, the user now creates templates,
• when the user requests a node from the registry, a new instance is created, but with function references from template -> start, finish, run
• if we run the tree again that uses registered nodes, it will use the previously created

Example:

local templates = {
  sail_to = {
      type = BehaviourTree.Task,
      run = function(task, ship_id)
         [code]
      end
  },
  sink_ship = {
      type = BehaviourTree.Sequence,
      nodes = {
          'sail_to',
          {
	      type = BehaviourTree.Task,
	      run = function(task, ship_id)
                  [code]
	      end
          },
      },
  },
}
BehaviourTree.register(templates)

[tanema]

Okay this all looks like it makes sense and is a good looking changeset. Can you add some unit tests to ensure that it does what is expected and will not break in the future?

[tanema]

Please rebase, I updated the testing for PRs from travis to github actions so that you can get automated feedback on your work.

[s-kania]

Okay this all looks like it makes sense and is a good looking changeset. Can you add some unit tests to ensure that it does what is expected and will not break in the future?

Yes, I will add tests and check if everything is OK.