Fix for cGy error in units conversion tab.

Fix some uploaded images.

New features added:
- In dose rate tab, now you can add shielding to your calculation (only Pb) and choose its units (mm or cm).
- Inverse Square Law tab was introduced for calculating radiation exposure at different distances.
- Walk through tutorial was introduced to easily understand the app.
- Better functionalities and design.

1. Some images fixed.
2. In Activity to dose rate tap: the result unit changed to (microSievert/hours).
3. App preview was uploaded on app page.