added get or add to the blackboard

[Clicky-coded]

This will close #435

[bitbrain]

@Clicky-coded could you extend the existing blackboard unit test to have a simple test for this?

[bitbrain]

Unit test required

[Clicky-coded]

I dont think i will be able to write a unit test for some time as I do not know gdunit and am busy with other stuff

[bitbrain]

@Clicky-coded it's fairly easy, see https://github.com/bitbrain/beehave/blob/godot-4.x/test/blackboard/blackboard_test.gd basically just 4-5 lines where you call the method and verify it works.

Unfortunately, I cannot accept pull requests without test coverage, as beehave has very high quality standards.