Support summing over lists of pixel masks

[samaloney]

Currently get_data supports summing over pixel from low to high e.g. get_data(pixel_indices=[[0,1],[2,3]]) would some pixel 0 and 1 and 2 and 3 while and pixel_indices=[[0,4]] would sum the first four pixel.

However this really isn't that useful as what this should support is summing pixel sets or set of pixel masks e.g to create the A (0, 4, 9), B (1, 5, 10), C (2, 6, 7) , D (3, 7, 11) meta pixel should be pixel_indices=[[0, 4, 8], [1, 5, 9], [2, 6, 10], [3, 7, 11]] or [ True, False, False, False, True, False, False, False, False, True, False, False] A and so on.