Implement Igusa invariants using lifts

[jvoight]

See Bruinier, "1-2-3 of Modular Forms", combine (3.4) with Theorem 3.35.

[jvoight]

Because the Doi-Naganuma lift respects the action of the Hecke algebra, I think @edgarcosta and @BenKBreen it's easy enough to understand this in terms of the Hecke eigenvalues and splitting behavior of primes in the quadratic extension.