RFE: move delete-instructions from compile -->to compute time

[ankostis]

Assuming my understanding is correct:
The delete-instructions reduce the memory footprint of cached results. They are pre-computed on network-construction (in compile()). They are inserted between operation nodes in the (Network.steps) attribute, independent of the DAG. The criteria to insert such a delete-instruction is if a need is not used by any other operation further down the DAG. The cache is not reused across compute() invocations, so any memory-reductions are for as long as a single computation runs.

On compute-time, the network is pruned based on:

• given input,
• requested output.

With #18 merged, unsatisfied operations working, it is quite probable that more delete-instructions can fit into the steps list after pruning. And it would make sense to re-ru it before each rerun.

[ankostis]

This is an example where the DeleteInstruction vary with different inputs, depending on whether b is given or not:

def test_deleteinstructs_vary_with_inputs():
    # Check #21: DeleteInstructions positions vary when inputs change.
    netop = compose(name="netop")(
        operation(name="a free without b", needs=["a"], provides=["aa"])(identity),
        operation(name="satisfiable", needs=["a", "b"], provides=["ab"])(add),
        operation(name="optional ab", needs=["aa", modifiers.optional("ab")], provides=["asked"])
        (lambda a, ab=10: a + ab),
    )

    inp = {"a": 2, "b": 3}
    exp = inp.copy(); exp.update({"aa": 2, "ab": 5, "asked": 7})
    res = netop(inp)
    assert res == exp  # ok
    steps11 = netop.net.steps
    res = netop(inp, outputs=["asked"])
    assert res == filtdict(exp, "asked")  # ok
    steps12 = netop.net.steps

    inp = {"a": 2}
    exp = inp.copy(); exp.update({"aa": 2, "asked": 12})
    res = netop(inp)
    assert res == exp  # ok
    steps21 = netop.net.steps
    res = netop(inp, outputs=["asked"])
    assert res == filtdict(exp, "asked")  # ok
    steps22 = netop.net.steps

    assert steps11 == steps12
    assert steps21 == steps22
    assert steps11 != steps21  # FAILs in v1.2.4 + #18, fixed by #26
    assert steps12 != steps22  # FAILs in v1.2.4 + #18, fixed by #26