Issue Description:
Convert functions in PrincipalTokenUtils have confusing argument _ibtUnit which is passed in the _computeYield. We can observe that in all the places where _convertToSharesWithRate and _convertToAssetsWithRate are used RayMath.RAY_UNIT is passed as _ibtUnit. But the _ibtRate will be in the ibt decimals, not in RAY.
Recommendation:
Rename _ibtUnit to ray or something more intuitive to the reader.
