chore(dept): move test data to flag demo

[YaoYinYing]

The package looks great. However, when install this from source, it requires test data MDAnalysisTests to be installed by default.

This PR moves the demo data dependency to a new extra flag demo, to prevent the demo date fetching if no requested.

Changes made in this Pull Request:

• Changed: MDAnalysisTests dependency to extra flag demo
• Added: Test case for pip installation w/ extras flag (dry run only, just check the output for MDAnalysisTests strings)
• Changed: Doc: Readme
• Changed: Installation section from notebook example

PS: you can check the installation by:

# This contains no test data
pip install 'ermsfkit @ git+https://github.com/YaoYinYing/ermsfkit@depts-remove-demo-dep'

# This will install the test data `MDAnalysisTests`
pip install 'ermsfkit[demo] @ git+https://github.com/YaoYinYing/ermsfkit@depts-remove-demo-dep'

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated? No
• Issue raised/referenced? No