Allow multiple input feature with the same value for gaussian processes
do not allow multiple feature for non-noisy data. Unittest for noisy data #3162
remove unused code
remvoe unused import
simpler assertion in gp test
fix sign error in gp + unit test #3180
small cosmetic fixes
Merge remote-tracking branch 'origin/gp_max_ll' into next

Conflicts:
	sklearn/gaussian_process/tests/test_gaussian_process.py