Coach as a library (#348)

* CoachInterface + tutorial

* Some improvements and typo fixes

* merge tutorial 0 and 4

* typo fix + additional tutorial changes

* tutorial changes

* added reading signals and experiment path argument

rl_coach/graph_managers/graph_manager.py

@@ -23,12 +23,10 @@ from typing import List, Tuple
 import contextlib
 
 from rl_coach.base_parameters import iterable_to_items, TaskParameters, DistributedTaskParameters, Frameworks, \
-    VisualizationParameters, \
-    Parameters, PresetValidationParameters, RunType
+    VisualizationParameters, Parameters, PresetValidationParameters, RunType
 from rl_coach.checkpoint import CheckpointStateUpdater, get_checkpoint_state, SingleCheckpoint, CheckpointState
 from rl_coach.core_types import TotalStepsCounter, RunPhase, PlayingStepsType, TrainingSteps, EnvironmentEpisodes, \
-    EnvironmentSteps, \
-    StepMethod, Transition
+    EnvironmentSteps, StepMethod, Transition
 from rl_coach.environments.environment import Environment
 from rl_coach.level_manager import LevelManager
 from rl_coach.logger import screen, Logger
@@ -123,6 +121,10 @@ class GraphManager(object):
         self.time_metric = TimeTypes.EpisodeNumber
 
     def create_graph(self, task_parameters: TaskParameters=TaskParameters()):
+        # check if create graph has been already called
+        if self.graph_creation_time is not None:
+            return self
+
         self.graph_creation_time = time.time()
         self.task_parameters = task_parameters