Add alt attributes to images
Add alt attributes to images

Travis tests with htmlproofer identified missing alt attributes
for images on this page.

Update img tags with suitable alt values where required.

SuperNEMO-DBD/SuperNEMO-DBD.github.io,SuperNEMO-DBD/SuperNEMO-DBD.github.io,SuperNEMO-DBD/SuperNEMO-DBD.github.io