Sets ODE solver to use for model
Recognized solvers and their corresponding NONMEM advans:
+----------------------------+------------------+
| Solver | NONMEM ADVAN |
+============================+==================+
| CVODES | ADVAN14 |
+----------------------------+------------------+
| DGEAR | ADVAN8 |
+----------------------------+------------------+
| DVERK | ADVAN6 |
+----------------------------+------------------+
| IDA | ADVAN15 |
+----------------------------+------------------+
| LSODA | ADVAN13 |
+----------------------------+------------------+
| LSODI | ADVAN9 |
+----------------------------+------------------+