Add medium [skip ci]
Add feature to initialize medium with refractive index. [skip ci]
Import constants like permittivity of free space, speed of light in vacuum and magnetic permeability from sympy.physics.units rather than using Symbols for them.
Add tests for medium
Add args test for Medium
Add note for the commented failing test
Remove unnecessary flag and change default params
Merge pull request #7580 from debugger22/fermat

Add optical medium