Tensor expression with canonicalization: part1

     
    It is implemented a tensor class with monoterm canonicalization
    and algebraic operations on tensors.
    
    As an application,
    check of identities involving the cyclic symmetry of the Riemann tensor
    
    ```
    >>> from sympy.tensor.tensor import TensorIndexType, tensor_indices, TensorS
    >>> Lorentz = TensorIndexType('Lorentz', dummy_fmt='L')
    >>> i, j, k, l = tensor_indices('i,j,k,l', Lorentz)
    >>> symr = TensorSymmetry(riemann_bsgs)
    >>> R4 = TensorType([Lorentz]*4, symr)
    >>> R = R4('R')
    >>> t = R(i,j,k,l)*(R(-i,-j,-k,-l) - 2*R(-i,-k,-j,-l))
    >>> riemann_cyclic(t)
    0
    ```