181 singleton design pattern

[JSteve0]

• Added singleton design pattern to the model
• Added tests to confirm
• Updated readme

closes #181

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🔴	Statements	36.46% (+0.53% 🔼)	132/362
🔴	Branches	41.54% (+0.91% 🔼)	54/130
🔴	Functions	30.77%	16/52
🔴	Lines	37.25% (+0.54% 🔼)	130/349

Test suite run success

20 tests passing in 4 suites.

Report generated by 🧪jest coverage report action from a26d25a6b0d638ddb624e9ddecc918117d85f862