Add energy conservation demo

[henry2004y]

• Add two demos of energy conservations
• Fix source code hide syntax

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6363678) 47.10% compared to head (d5bb50a) 47.10%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #94 +/- ## ======================================= Coverage 47.10% 47.10% ======================================= Files 9 9 Lines 535 535 ======================================= Hits 252 252 Misses 283 283 ```

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