Improve the log of GDPOpt

[ZedongPeng]

Summary

There are four solver configurations defined in GDPOpt, including mip_solver, nlp_solver, minlp_solver and local_minlp_solver.

Sometimes, it makes me confused about which solver is used for the master problem or subproblems. It would be better if we could report which solver is used for each subproblem.
Attached is the log from DICOPT for reference.

 Major Major     Objective    CPU time  Itera- Evaluation Solver
 Step  Iter       Function     (Sec)    tions   Errors
  NLP    1      -693.29360      2.02      121      0      ipopth
  MIP    1      -555.90425      5.87    16289      0      cplex
  NLP    2       -77.45605<    11.40      683      0      ipopth
  MIP    2      -171.54170      7.63    30078      0      cplex
  NLP    3       -53.95034      2.62      178      0      ipopth