Criminal record and work on federally funded project
I was not able to find any documentation directly related to the NSF, however, I would find it unlikely that it would be a problem.

If nothing relevant is mentioned in your employment agreement/contract, then you are probably fine from a legal/regulatory perspective.

As for the ethical question of whether you should volunteer the information to your employer, I can say that if your performance on the NSF grant were contingent on criteria like your criminal history, the professors would have likely asked you directly during your interview.  If they did not feel that it was important enough to interrogate you about it, it probably does not matter either way.

Where it might be a problem is if the grant requires you get a security clearance, as felonies usually invalidate candidacy for a clearance.  It is very unlikely that a clearance would be required for NSF grants; it's more likely from funding agencies like DARPA and ONR.  With that said, grants don't last forever, and the professors might eventually get one that requires you to get a clearance.  If the professors thought that might be a possibility or concern, though, they would have likely brought it up during your interview.  Also, the government often makes exceptions for software engineers/computer scientists since there is a shortage of ones who are otherwise eligible for a clearance.