vtkMath: don't use x/y/z as argument names to avoid ambiguity

This also harmonizes the variable names in different functions (some used a/b/c while others used x/y/z), and between the functions and their documentation (some of them correctly referred to the input vectors as a and b, but referred to the output vector as z).

In the case of vtkMathPerpendiculars I used v1/v2/v3 because a/b/c is already used in the function body.