change_base_ring does not work for external packages

[wbhart]

Because change_base_ring uses the PolynomialRing constructor, it always generates a generic polynomial instead of creating a polynomial belonging to the module in use (e.g. Singular.jl).

I have no idea whatsoever how to fix this problem. Ideas are welcome.

[wbhart]

The only solution seems to be to pass the polynomial ring to change_base_ring. I don't see any other way around it.

[heiderich]

My suggestion in #106 (comment) was to hand over an algebra morphism, which should in particular specify the target ring. I suggested a pair (S, f), where S is the target ring and f the ring homomorphism from the base ring of the polynomial to S. Whether one needs a type for ring homomorphism, I am not sure. Maybe one could just add the target as an additional parameter.

[wbhart]

Now see the more comprehensive ticket #490