Add explicit search for Conway polys if not in database

[mhostetter]

I have determined the tests to test/check the Conway polynomial conditions, so there's no reason not to add it.

Option 1

Instead of raising a LookupError, perhaps we should embark on searching for the Conway polynomial. We can use Poly.is_conway() from #465. Since this will likely take an eternity, we should emit a warning telling the user.

Option 2

Add a kwarg flag to opt-in to the fallback manual search. There could be a benefit to quickly returning with a LookupError by default.

[mhostetter]

Added in v0.3.3.