Rename method `average` to `integrate`

  `integrate` is a more accurate name for a function that integrates
   the spectral density. Docstrings and tutorial updated.
8 jobs for kpm_integral in 30 minutes and 11 seconds (queued for 2 seconds)
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manual build and upload conda package (manual) #15443
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Name Stage Failure
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check for broken links in doc Test There is an unknown failure, please try again
(line   27) redirect  http://prl.aps.org/abstract/PRL/v90/i25/e256601 - with Found to https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.90.256601
writing output... [100%] tutorial/superconductors


build finished with problems, 8 warnings.
Makefile:114: recipe for target 'linkcheck' failed
make: Leaving directory '/builds/pablopiskunow/kwant/doc'
make: *** [linkcheck] Error 1
ERROR: Build failed: exit code 1