Fix issue in PCG solver in algebraic_connectivity

Search direction p was aliased to iterate x when preconditioner M was
None.
Merge branch 'fiedler' into fiedler_nochol
Simplify TraceMIN main loop in algebraic_connectivity
Remove gram_schmidt from TrainMIN in algebraic_connectivity
Merge branch 'fiedler' into fiedler_nochol

Conflicts:
	networkx/linalg/algebraic_connectivity.py
Revert "Remove Cholesky solver in algebraic_connectivity"

This reverts commit 1be3dd85db68865c5cbd512b88a0dc03f32cdf36.

Conflicts:
	.travis.yml
	networkx/linalg/algebraic_connectivity.py