Incorporate crtalgsqrt into cado-nfs.py

I've done a few small SNFS factorizations recently. The problem of using a polynomial with Galois group ≅ C₂ × C₂ happened frequently to me, so that the sqrt step failed because there are no inert primes. The crtalgsqrt can be used in this case to compute the square root on the algebraic side, but this requires the user to run both sqrt and crtalgsqrt manually, copy-paste the respective square roots into a calculator and compute gcd(sa-sr,N).

It would be nice to include crtalgsqrt into cado-nfs.py. At the start of the sqrt step, the polynomials can be checked whether they both work with the sqrt program and, if not, crtalgsqrt can be called for taking the square root in the offending number fields instead. The gcd() could be computed in Python.