Review structure of merged inverse squareroot algorithms

[SamFrengley]

Review the definitions of the algorithms:

• inverse_and_sqrt
• inv_sqrt
• invertme_sqrtother
• sqrt_ratio

At the moment they are implemented so that we can do

a.inverse_and_sqrt();                      // 1/a, sqrt(a)
a.inv_sqrt();                                      // sqrt(1/a)
a.invertme_sqrtother(&b);            // 1/a, sqrt(b) 
a.sqrt_ratio(&b);                              // sqrt(a/b)

Maybe it is better for the latter two to be implemented as

FieldOps::invertme_sqrtother(&a, &b);
FieldOps::sqrt_ratio(&a, &b);