Rejigged solver statistics in preparation for islanding.

- See public description below.
- Stopped incrementing the iteration count in saveStats().
- `sizeof(mjSolverStat) == 40`, so this ends up costing 160KB, up from 40KB.

BEGIN_PUBLIC

Changed the size of `mjData.solver`, the structure used to collect solver diagnostic information. The array is now of length `mjNISLAND * mjNSOLVER`, where each row of length `mjNSOLVER` contains separate solver statistics for each constraint island. Until solver islanding is implemented, only row 0 is used.

- The new constant `mjNISLAND` is set to 20.
- `mjNSOLVER` is reduced from 1000 to 200.
- Added `mjData.solver_nisland`, the number of islands for which the solver ran.
- `mjData.solver_niter` (renamed from mjData.solver_iter) and `mjData.solver_nnz` are now integer vectors of length `mjNISLAND`.

END_PUBLIC

PiperOrigin-RevId: 565030093
Change-Id: I773e918805c6ced79f0dab5f19ea23956760c8c5

include/mujoco/mjdata.h

@@ -173,9 +173,10 @@ struct mjData_ {
   mjTimerStat   timer[mjNTIMER];      // timer statistics
 
   // solver statistics
-  mjSolverStat  solver[mjNSOLVER];  // solver statistics per iteration
-  int     solver_iter;              // number of solver iterations
-  int     solver_nnz;               // number of non-zeros in Hessian or efc_AR
+  mjSolverStat  solver[mjNISLAND*mjNSOLVER];  // solver statistics per island, per iteration
+  int     solver_nisland;           // number of islands processed by solver
+  int     solver_niter[mjNISLAND];  // number of solver iterations, per island
+  int     solver_nnz[mjNISLAND];    // number of non-zeros in Hessian or efc_AR, per island
   mjtNum  solver_fwdinv[2];         // forward-inverse comparison: qfrc, efc
 
   // collision statistics