Goldsby (2006) rheology

[grinsted]

Description

The parameters in https://github.com/pism/pism/blob/main/src/rheology/GoldsbyKohlstedt.cc are based on their 2001 paper.

However - shortly after (2006) Goldsby published a revised set of parameters:
"Superplastic Flow of Ice Relevant to Glacier and Ice-Sheet Mechanics"
https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470750636.ch60

I propose that this set of parameters should be used instead.

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NOTE: One issue to be aware of is that with the parameters in Goldsby2006 then A(T) is discontinous at the critical temperature 258K. That clearly cannot be right. I therefore believe that the threshold should also be revised to be at the crossover point where A_cold(T) == A_warm(T). However, with that set of parameters A_cold is always above A_warm.