Unexpected behavior when using xpress.sum over numpy array slice of xpress variables

Hi Thomas,

Which versions of NumPy and the Xpress Python interface are you using? I can't reproduce this using NumPy 1.19.0 and the latest Xpress version, 9.0.3:

>>> import numpy as np
>>> x = xp.vars(5, 5, 5, name = 'x')
>>> sum = np.array([xp.Sum([x[:, :, j] for j in range(5)])], dtype=xp.npexpr)
>>> sum = np.squeeze(sum)
>>> print(sum)
[[  x(0,0,0) + x(0,0,1) + x(0,0,2) + x(0,0,3) + x(0,0,4)
    x(0,1,0) + x(0,1,1) + x(0,1,2) + x(0,1,3) + x(0,1,4)
    x(0,2,0) + x(0,2,1) + x(0,2,2) + x(0,2,3) + x(0,2,4)
    x(0,3,0) + x(0,3,1) + x(0,3,2) + x(0,3,3) + x(0,3,4)
    x(0,4,0) + x(0,4,1) + x(0,4,2) + x(0,4,3) + x(0,4,4)]
 [  x(1,0,0) + x(1,0,1) + x(1,0,2) + x(1,0,3) + x(1,0,4)
    x(1,1,0) + x(1,1,1) + x(1,1,2) + x(1,1,3) + x(1,1,4)
    x(1,2,0) + x(1,2,1) + x(1,2,2) + x(1,2,3) + x(1,2,4)
    x(1,3,0) + x(1,3,1) + x(1,3,2) + x(1,3,3) + x(1,3,4)
    x(1,4,0) + x(1,4,1) + x(1,4,2) + x(1,4,3) + x(1,4,4)]
 [  x(2,0,0) + x(2,0,1) + x(2,0,2) + x(2,0,3) + x(2,0,4)
    x(2,1,0) + x(2,1,1) + x(2,1,2) + x(2,1,3) + x(2,1,4)
    x(2,2,0) + x(2,2,1) + x(2,2,2) + x(2,2,3) + x(2,2,4)
    x(2,3,0) + x(2,3,1) + x(2,3,2) + x(2,3,3) + x(2,3,4)
    x(2,4,0) + x(2,4,1) + x(2,4,2) + x(2,4,3) + x(2,4,4)]
 [  x(3,0,0) + x(3,0,1) + x(3,0,2) + x(3,0,3) + x(3,0,4)
    x(3,1,0) + x(3,1,1) + x(3,1,2) + x(3,1,3) + x(3,1,4)
    x(3,2,0) + x(3,2,1) + x(3,2,2) + x(3,2,3) + x(3,2,4)
    x(3,3,0) + x(3,3,1) + x(3,3,2) + x(3,3,3) + x(3,3,4)
    x(3,4,0) + x(3,4,1) + x(3,4,2) + x(3,4,3) + x(3,4,4)]
 [  x(4,0,0) + x(4,0,1) + x(4,0,2) + x(4,0,3) + x(4,0,4)
    x(4,1,0) + x(4,1,1) + x(4,1,2) + x(4,1,3) + x(4,1,4)
    x(4,2,0) + x(4,2,1) + x(4,2,2) + x(4,2,3) + x(4,2,4)
    x(4,3,0) + x(4,3,1) + x(4,3,2) + x(4,3,3) + x(4,3,4)
    x(4,4,0) + x(4,4,1) + x(4,4,2) + x(4,4,3) + x(4,4,4)]]
>>> np.version.full_version
'1.19.0'
>>> xp.__version__
'9.0.3'