Yes, it is classed as a disability in the UK and I think also in the USA. This is definitely discrimination but the question is whether you want to take them to court over it. If I were you I would probably let this one go and develop a different strategy for the next interview. The key is to make them realise that you are excellent at what you are doing in a lab and that they should value that instead of picking on your speech. I would go into the next interview not trying to hide your stammer (because that will invariably make it worse) but disclosing it as the very first thing you say. Tell them that you have a stammer, that you are not nervous but sometimes need more time to say things, and that this does not affect the quality of your work in any way. I think this proactive approach would impress an interviewer.