Added example conservation

[brosaplanella]

Description

Added an example script for lithium conservation.

Fixes #1185

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s. If necessary, also add to the list of breaking changes.

• New feature (non-breaking change which adds functionality)

Key checklist:

• No style issues: $ flake8
• All tests pass: $ python run-tests.py --unit
• The documentation builds: $ cd docs and then $ make clean; make html

You can run all three at once, using $ python run-tests.py --quick.

Further checks:

• Code is commented, particularly in hard-to-understand areas

[brosaplanella]

Does this need a CHANGELOG entry?

[rtimms]

thanks @ferranbrosa ! I guess you could add that there is a new example to the changelog, but no big deal if not.