MAINT remove default values from private funcs in GMM

Removed default values for 'mean' and 'covars' arguments.
They did not have the excepted format causing the functions
to fail when called with default arguments.
ENH faster sampling in GMMs

A few optimizations for sampling with full covariance matrix:
1) use eigh instead of svd, which are equivalent (up to numerical precision),
2) use broadcasting instead of matrix multiplication for the product U s^(1/2),
3) simpler construction of the random vectors.
Merge branch 'pr/3120'

Fixes #3120, "Bad default arguments in gmm.py".