Select spins with signal to noise ratio higher or lower than the given ratio.
ratio: The signal to noise ratio to compare to.
operation: The comparison operation by which to select the spins.
all_sn: A flag specifying if all the signal to noise ratios per spin should match the comparison operator, of if just a single comparison match is enough.
The comparison operation is the method which to select spins according to: operation(sn_ratio, ratio).
The possible operations are: `<':strictly less than, `<=':less than or equal, `>':strictly greater than, `>=':greater than or equal, `==':equal, `!=':not equal.
The `all_sn' flag default is True, meaning that if all of the spin's signal to noise levels evaluates to True in the comparison, the spin is selected.
To select all spins with a signal to noise ratio higher than 10.0:
[numbers=none] relax> select.sn_ratio(ratio=10.0, operation='>')
[numbers=none] relax> select.sn_ratio(ratio=10.0, operation='>', all_sn=False)