Add new 3to4 conversion rules to change sign of poloidal flux

[maarten-ic]

Add a list of quantities that should get a sign flip on conversion, but are not covered by the generic rule that looks at DD3 cocos_label_transformation metadata.

Fixes #61

[olivhoenen]

LGTM, but maybe @imbeauf can check and confirm the list of paths for sign flip?
E.g. is ggd/psi/coefficients also affected?

[imbeauf]

I confirm the list, but for the GGD coefficients ggd/psi/coefficients. We have no a priori definition of these coefficients, so there is no guarantee that changing their sign will simply change the sign of the evaluated psi.
I would suggest not to introduce any transformation for those, to avoid the risk of introducing potential errors.