Update dependencies: replace DoubleFloat by Quadmath

Matthieu Ancellinrequested to merge
mancellin/shaman_julia:dep into master

The DoubleFloat package is actually unused in the current version: the Float128 type comes actually for Quadmath, one of DoubleFloat's dependencies.

I removed DoubleFloat, but we could also replace Float128 by DoubleFloat64 which actually comes from DoubleFloat.

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