add gifs showing features of xolotl in readme

[sg-s]

• [ ] gif showing optimization using xfit
• [ ] gif showing xgrid functionality
• [x] gif showing manipulate
• [ ] gif showing click-through ability
• [ ] gif showing frozen traces during manipulate

just lots of gifs. need to be gifs because anything else won't load on github readme

[github-actions[bot]]

Stale issue message