Rename qiReduce to qiSimplify

There is a meaning for a reduced QI and this is not it.
Rename Rational qi{Add,Sub,Mul,Div} as qiAddR etc.
qi constructor: Special case √1
Add qiNegate
Tests: use CReal for precision
Add qiAdd, qiSub, qiMul, qiDiv
Implement qiPow in terms of qiMul