OPT: avoid argsort by only storing forward indices during tractography

3 jobs for master in 8 minutes and 50 seconds (queued for 1 second)
Name Stage Failure
failed
test37 Test
    req = request.PositionRequest(sp.zeros((1, 3)))

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=========================== short test summary info ============================
FAILED gyral_structure/test/test_sumbase.py::test_sumbase - ValueError: opera...
========= 1 failed, 20 passed, 1 deselected, 329464 warnings in 44.78s =========
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ERROR: Job failed: exit code 1
failed
test36 Test
    req = request.PositionRequest(sp.zeros((1, 3)))

-- Docs: https://docs.pytest.org/en/latest/warnings.html
=========================== short test summary info ============================
FAILED gyral_structure/test/test_sumbase.py::test_sumbase - ValueError: opera...
========= 1 failed, 20 passed, 1 deselected, 329464 warnings in 41.60s =========
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ERROR: Job failed: exit code 1
failed
test35 Test
      File "/tmp/pip-install-4_f92g5o/llvmlite/ffi/build.py", line 109, in main_posix
"to the path for llvm-config" % (llvm_config,))
RuntimeError: llvm-config failed executing, please point LLVM_CONFIG to the path for llvm-config
error: command '/usr/local/bin/python' failed with exit status 1
----------------------------------------
ERROR: Command errored out with exit status 1: /usr/local/bin/python -u -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-4_f92g5o/llvmlite/setup.py'"'"'; __file__='"'"'/tmp/pip-install-4_f92g5o/llvmlite/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' install --record /tmp/pip-record-ry0lfum9/install-record.txt --single-version-externally-managed --compile --install-headers /usr/local/include/python3.5m/llvmlite Check the logs for full command output.
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ERROR: Job failed: exit code 1