Bug in summing "LinOpConv"s with different indices

[ThomasDeb]

I encountered a bug when trying to sum two "LinOpConv" operators whose dimensions along which the convolution is performed do not match. I'm attaching a code snippet illustrating the problem. I don't think it is possible to rewrite this sum as a single convolution operator in the general case, so an easy fix would be to change the "plus_" method of LinOpConv accordingly (from testing isa(G,'LinOpConv') to something like isa(G,'LinOpConv') && isequal(sort(this.index), sort(G.index))). However I'm not sure I fully understand all the subcases of the "plus_" method so I'm not confortable trying to fix the problem completely myself

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