move generator.py to tests

fix on 18.04:
Traceback (most recent call last):
  File ".../tests/python/test_Tasks.py", line 268, in <module>
    from models.arm.generator import create_7dof_arm
ImportError: No module named models.arm.generator

Author: nim65s

models/arm/generator.py renamed to tests/python/generator.py

@@ -262,10 +262,7 @@
 print("")
 
 # Get robot model generator module
-import sys
-filepath = os.path.normpath(os.path.join(os.path.dirname(os.path.abspath(__file__)), '../../'))
-sys.path.append(str(filepath))
-from models.arm.generator import create_7dof_arm
+from generator import create_7dof_arm
 
 # Get robot model
 model, geom_model = create_7dof_arm() # A robot containing sperical joints where nq != nv