Power method tolerance 1e-5 should be smaller

[MargaretDuff]

Description

The default tolerance for the power method, used to calculate the norm of linear operators, is set to 1e-5. This is used when deciding if the calculated norm is small enough to be assumed to be zero and prevents the algorithm from diverging by dividing by zero.

CIL/Wrappers/Python/cil/optimisation/operators/Operator.py

Lines 339 to 345 in 5d569dc

	x0_norm = x0.norm()
	if x0_norm < tolerance:
	log.warning(
	"The operator has at least one zero eigenvector and is likely to be nilpotent")
	eig_new = 0.
	break
	x0 /= x0_norm

@bosschmidt found that this incorrectly returned his operator's norm to zero, and setting the tolerance to 1e-8 eventually led to the power method calculating a correct non-zero norm value.

Perhaps this tolerance needs re-thinking.

[MargaretDuff]

We will investigate this but in the meantime we should at least change the warning