Missing conversions from GFqDom to ZRing<Element>

[ClementPernet]

Follow up of #160 . GFqDom is a tabulated implementation of Galois field with init and convert of elements to/from double allowing some slack so as to perform small dot products with no overflow. In order to exploit it in fflas, one need to implement conversion methods (using ModeTrait::ConvertTo) to convert from GFqDom to ZRing<double>.