Ensure positive fractional tune calculated from transfer matrix

[austin-hoover]

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[austin-hoover]

sns_ring_tunes.zip

Here's an SNS ring example. The tunes are:

nux = 6.710221077829864
nuy = 6.732402637896412

But the reported fractional tunes are:

 'fractional tune x': -0.28977892148070594,
 'fractional tune y': -0.26759736210364865,

With this PR the fractional tunes are:

 'fractional tune x': 0.7102210785192941,
 'fractional tune y': 0.7324026378963513,