Varie

• The set of variables to be solved is now implicitly determined from all initial guesses (`≈`);
• The function signature is now more flexible. Users can either omit the variable list to get all results, or specify which variables they want returned;
• Output variables can be specified either as a flat list (`find(sys, x, y)`) or as a `sys`/`mat` block (`find(sys, sys(x,y))`);
• Introduces `_` as a wildcard argument (`find(sys, _)` or `MathcadBlock -> find(_)`);
• The output is now filtered to return only the subset of variables requested by the user, in the specified order.