Fixes #685, fixes #31

Summary/Motivation:

This is the initial step toward integrating numpy with the Pyomo expression system. This provides basic support for generating Pyomo expressions contained in NumPy ndarrays.

It is by no means complete (there will be at least 3 more parts to finish the integration), but I felt breaking this up into chunks will make it easier to review. However, this PR will support the following example:

>>> import numpy as np
>>> from pyomo.environ import *
>>> m = ConcreteModel()
>>> m.x = Var([0,1,2,3])
>>> A = np.array([[1, 2, 3, 4], [5, 6, 7, 8]])
>>> b = np.array([10, 20])
>>> m.c = Constraint([0,1], expr=A @ m.x <= b)
>>> m.pprint()
2 Set Declarations
    c_index : Size=1, Index=None, Ordered=Insertion
        Key  : Dimen : Domain : Size : Members
        None :     1 :    Any :    2 : {0, 1}
    x_index : Size=1, Index=None, Ordered=Insertion
        Key  : Dimen : Domain : Size : Members
        None :     1 :    Any :    4 : {0, 1, 2, 3}

1 Var Declarations
    x : Size=4, Index=x_index
        Key : Lower : Value : Upper : Fixed : Stale : Domain
          0 :  None :  None :  None : False :  True :  Reals
          1 :  None :  None :  None : False :  True :  Reals
          2 :  None :  None :  None : False :  True :  Reals
          3 :  None :  None :  None : False :  True :  Reals

1 Constraint Declarations
    c : Size=2, Index=c_index, Active=True
        Key : Lower : Body                              : Upper : Active
          0 :  -Inf :   x[0] + 2*x[1] + 3*x[2] + 4*x[3] :  10.0 :   True
          1 :  -Inf : 5*x[0] + 6*x[1] + 7*x[2] + 8*x[3] :  20.0 :   True

4 Declarations: x_index x c_index c

Please note that this implementation is NOT intended to be "numpy-fast": we still generate the same expanded Pyomo expression trees that we always have. However, this is intended to lay the groundwork for a fast "matrix-based" expression system later.

Changes proposed in this PR:

• add support for casting some IndexedComponents into ndarray objects
• add recognition for ndarray in Initializer()

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