solutions to calling julia from singular kernel

[tthsqe12]

Singular.jl still cannot factor over Q(alpha). The best solution seems to be:

• at Singular.jl initialization, get a function pointer to the corresponding julia function, and
• tell singular about this function pointer with a new singular kernel function (requires coordination with @hannes14 )

We need to intercept the polynomial before it gets to factory, because we can't do anything with it from julia once it is in factory.

There probably will be many julia functions that the singular kernel will want to use in the future, so maybe the process of adding one can be as painless as possible?