Performance of writing matrixtable on 0.2

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for now you can also get down to O(N * log2 N) doing a tree union:

mts_ = mts[:]

iteration = 0
while (len(mts_) > 1):
    iteration += 1
    print(f'iteration {iteration}')
    tmp = []
    for i in range(0, len(mts_), 2):
        tmp.append(hl.MatrixTable.union_cols(mts_[i:i+2]))
    mts_ = tmp[:]
[final_mt] = mts_