Refactor/transfer mechanism/mutiple output states (#535)

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* Merge branch 'devel' of https://github.com/PrincetonUniversity/PsyNeuLink into CURRENT

# Conflicts:
#	tests/mechanisms/test_transfer_mechanism.py

* Merge branch 'devel' of https://github.com/PrincetonUniversity/PsyNeuLink into CURRENT

# Conflicts:
#	tests/mechanisms/test_transfer_mechanism.py

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* • State
  - docstring: STATE NAME ENTRY, MECHANISM/STATE and State constructor examples

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* • TransferMechanism
  - instantiate one OutputState for each item of variable

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* • TransferMechanism
  - _instantiate_output_states:  added to implement one standard OutputState for each InputState
• OutputState
  - fixed bug in assigning params to standard OutputStates

* • Mechanism subclasses
  - removed specification of output_states as list (replaced with string or tuple
    (to avoid mutable default assignment)
• TransferMechanism
  - __init__: assigned output_states to RESULT rather than [RESULT] to avoid default "gotcha"
  - _instantiate_output_states:  added to implement one standard OutputState for each InputState
• OutputState
  - fixed bug in assigning params to standard OutputStates

* • Mechanism subclasses
  - removed specification of output_states as list (replaced with string or tuple
    (to avoid mutable default assignment)
• TransferMechanism
  - __init__: assigned output_states to RESULT rather than [RESULT] to avoid default "gotcha"
  - _instantiate_output_states:  added to implement one standard OutputState for each InputState
• OutputState
  - fixed bug in assigning params to standard OutputStates

* • Mechanism subclasses
  - removed specification of output_states as list (replaced with string or tuple
    (to avoid mutable default assignment)
• TransferMechanism
  - __init__: assigned output_states to RESULT rather than [RESULT] to avoid default "gotcha"
  - _instantiate_output_states:  added to implement one standard OutputState for each InputState
• OutputState
  - fixed bug in assigning params to standard OutputStates