Hydra skip

[brettviren]

This is to try to solve #148 by finally finishing support for the "hydra" flow graph node category.

[HaiwangYu]

Thanks @brettviren, this would be very useful once available.

As we chatted, I want to make a note here for your debugging.
jsonnet: https://www.phy.bnl.gov/~yuhw/dunefd/hydra-skip/wct-sim-fans-hydra.jsonnet
vis: https://www.phy.bnl.gov/~yuhw/dunefd/hydra-skip/wct-sim-fans-hydra.pdf
error with tbb: https://www.phy.bnl.gov/~yuhw/dunefd/hydra-skip/hydra-tbb.log

[brettviren]

@HaiwangYu I added test_hydra_input.cxx to try to exercise the hydra front end in a stand alone way. So far it only seems to run the last out of two sources. Maybe the test is wrong or maybe hydra is.

[HaiwangYu]

Hi @brettviren, if you are OK, I want to resolve the conflict and merged this PR.

[brettviren]

@HaiwangYu that would be very welcome, thank you.