Improve homogenization step in factoring

[axkr]

Improve the homogenization step in the factoring algorithm:

• https://www.research.ed.ac.uk/portal/files/413486/Solving_Symbolic_Equations_%20with_PRESS.pdf

See the homog_ methods in the Prolog repository:

• https://github.com/maths/PRESS/tree/master/pressdir/methods

Here you can find the current implementation:

• https://github.com/axkr/symja_android_library/blob/master/symja_android_library/matheclipse-core/src/main/java/org/matheclipse/core/polynomials/PolynomialHomogenization.java

Usage:

...
PolynomialHomogenization substitutions =
          new PolynomialHomogenization(eVar.getVarList(), engine);
      IExpr subsPolynomial = substitutions.replaceForward(expr);
      if (substitutions.size() == 0) {
// no substitution
        return ... ;
      }
      if (subsPolynomial.isAST()) {
        eVar.addAll(substitutions.substitutedVariablesSet());
        IExpr factorization = ...;
        if (factorization.isPresent()) {
          return substitutions.replaceBackward(factorization);
        }
      }