Add support for object arrays about sparse HOT 2 OPEN

import sparse
from sympy.abc import a, b
aa = sparse.COO([[0],[0],[0]], [a], shape=(1,1,1))
bb = sparse.COO([[0],[0],[0]], [b], shape=(1,1,1))
aa + bb

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-37-6b05462dc5e0> in <module>
----> 1 aa + bb

~/anaconda/envs/symengine/lib/python3.7/site-packages/numpy/lib/mixins.py in func(self, other)
     23         if _disables_array_ufunc(other):
     24             return NotImplemented
---> 25         return ufunc(self, other)
     26     func.__name__ = '__{}__'.format(name)
     27     return func

~/anaconda/envs/symengine/lib/python3.7/site-packages/sparse/coo/core.py in __array_ufunc__(self, ufunc, method, *inputs, **kwargs)
   1439 
   1440         if method == '__call__':
-> 1441             result = elemwise(ufunc, *inputs, **kwargs)
   1442         elif method == 'reduce':
   1443             result = COO._reduce(ufunc, *inputs, **kwargs)

~/anaconda/envs/symengine/lib/python3.7/site-packages/sparse/coo/umath.py in elemwise(func, *args, **kwargs)
     45     """
     46 
---> 47     return _Elemwise(func, *args, **kwargs).get_result()
     48 
     49 

~/anaconda/envs/symengine/lib/python3.7/site-packages/sparse/coo/umath.py in __init__(self, func, *args, **kwargs)
    420         self.cache = {}
    421 
--> 422         self._get_fill_value()
    423         self._check_broadcast()
    424 

~/anaconda/envs/symengine/lib/python3.7/site-packages/sparse/coo/umath.py in _get_fill_value(self)
    465         from .core import COO
    466 
--> 467         zero_args = tuple(arg.fill_value[...] if isinstance(arg, COO) else arg for arg in self.args)
    468 
    469         # Some elemwise functions require a dtype argument, some abhorr it.

~/anaconda/envs/symengine/lib/python3.7/site-packages/sparse/coo/umath.py in <genexpr>(.0)
    465         from .core import COO
    466 
--> 467         zero_args = tuple(arg.fill_value[...] if isinstance(arg, COO) else arg for arg in self.args)
    468 
    469         # Some elemwise functions require a dtype argument, some abhorr it.

TypeError: 'int' object is not subscriptable