Added singularity def file to ease deployment

[SteampunkIslande]

Hello,
Thank you for your amazing web app, users from the wet lab really enjoy it :)

Here, I'm submitting a pull request for two main reasons:

First, the website that hosts your streamlit app is (currently) down, and I was asked by my supervisor to make it available through our local network (so that if the app is down, I take the blame ^^).

Second, I really struggled turning this app into a singularity image file, because it requires the whole sequana installation when only requiring two files to check the samplesheet.

So here it is, I changed the code so it does not depend on sequana, hope you don't mind.

I tested it on my Linux box and it works perfectly fine (as long as $HOME and /tmp are writable file systems).