ENH: improve expression string expansions

- reuse more of stringOps expansions to reduce code and improve the
  syntax flexiblity.

  We can now embed "pre-calculated" values into an expression.
  For example,

       angle       35;
       valueExpr   "vector(${{cos(degToRad($angle))}}, 2, 3)";

  and the ${{..}} will be evaluated with the regular string evaluation
  and used to build the entire expression for boundary condition
  evaluation.

  Could also use for fairly wild indirect referencing:

       axis1   (1 0 0);
       axis2   (0 1 0);
       axis3   (0 0 1);
       index   100;
       expr   "$[(vector) axis${{ ($index % 3) +1 }}] / ${{max(1,$index)}}";