Bump version to v0.3.1, add changelog and add modifiers test

[TheGupta2012]

Summary of changes

• Added support for ctrl and negctrl modifiers with pyqasm>=0.2.1.
• Updated pyqasm dependency from "pyqasm>=0.1.0,<0.2.0" to "pyqasm>=0.2.1,<0.3.0". [1] [2]
• Updated version 0.3.1 across multiple files
• Update CHANGELOG.md

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

📢 Thoughts on this report? Let us know!