improve lll and voronoi algorithms in kwant.linalg

Improve the logic of 'cvp', now it provably works in all cases.
Add 'reduced' option to 'linalg.lll.voronoi', this eliminates
superfluous Voronoi vectors that do not have a corresponding face
of the Voronoi cell, or this face would be smaller than 'rtol'.

Simplify 'lattice.Polyatomic.neighbors' by providing the option
to group equidistant vectors 'group_by_length' in 'linalg.lll.cvp'.

See !165 for details.

Closes #163.
13 jobs for master in 16 minutes and 24 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #37596
build package

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allowed to fail
check whitespace style

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  Test
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build PDF documentation

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passed #37599
build documentation

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failed #37602
allowed to fail
check for broken links in doc

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passed #37598
allowed to fail
check for dependencies installed

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passed #37601
run tests

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  Deploy
passed #37603
build and upload conda package

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manual #37605
allowed to fail manual
remove_coverage
manual #37607
allowed to fail manual
remove_docs
passed #37604
upload coverage

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passed #37608
upload dev version docs

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passed #37606
upload documentation to the test server

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Name Stage Failure
failed
check for broken links in doc Test
(line   39) broken    http://www.numpy.org/ - HTTPConnectionPool(host='www.numpy.org', port=80): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x7f4d79eb6550>: Failed to establish a new connection: [Errno -2] Name or service not known',))
writing output... [100%] tutorial/superconductors


build finished with problems, 151 warnings.
make: *** [linkcheck] Error 1
Makefile:119: recipe for target 'linkcheck' failed
make: Leaving directory '/builds/kwant/kwant/doc'
ERROR: Job failed: exit code 1