OPT: don't recompile cuda code for every tractography step

Achieved by calling update_pos to update the positions
in the GPU arrays (as well as updating the indices in the
forward and backwards arrays)
3 jobs for master in 8 minutes and 18 seconds
Name Stage Failure
failed
test36 Test
FAILED gyral_structure/test/test_cost.py::test_costfunc - NotImplementedError...
FAILED gyral_structure/test/test_cost.py::test_param_cost - NotImplementedErr...
FAILED gyral_structure/test/test_radial_basis.py::test_precompute - ValueErro...
FAILED gyral_structure/test/test_streamline.py::test_spiral_basis - Attribute...
FAILED gyral_structure/test/test_sumbase.py::test_sumbase - ValueError: Reque...
========= 6 failed, 15 passed, 1 deselected, 19447 warnings in 21.17s ==========
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ERROR: Job failed: exit code 1
failed
test35 Test
      File "/tmp/pip-install-g7gj1ict/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-g7gj1ict/llvmlite/setup.py'"'"'; __file__='"'"'/tmp/pip-install-g7gj1ict/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-vpffbciw/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
failed
test37 Test
FAILED gyral_structure/test/test_cost.py::test_costfunc - NotImplementedError...
FAILED gyral_structure/test/test_cost.py::test_param_cost - NotImplementedErr...
FAILED gyral_structure/test/test_radial_basis.py::test_precompute - ValueErro...
FAILED gyral_structure/test/test_streamline.py::test_spiral_basis - Attribute...
FAILED gyral_structure/test/test_sumbase.py::test_sumbase - ValueError: Reque...
========= 6 failed, 15 passed, 1 deselected, 19447 warnings in 18.81s ==========
Running after_script
Uploading artifacts for failed job
ERROR: Job failed: exit code 1