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

src/engine/engine_core_smooth.c

@@ -1212,11 +1212,17 @@ void mj_solveM(const mjModel* m, mjData* d, mjtNum* x, const mjtNum* y, int n) {
 // in-place sparse backsubstitution for one island:  x = inv(L'*D*L)*x
 //  L is in lower triangle of qLD; D is on diagonal of qLD
 void mj_solveM_island(const mjModel* m, const mjData* d, mjtNum* restrict x, int island) {
+  // if no islands, call mj_solveLD
+  const mjtNum* qLD = d->qLD;
+  const mjtNum* qLDiagInv = d->qLDiagInv;
+  if (island < 0) {
+    mj_solveLD(m, x, 1, qLD, qLDiagInv);
+    return;
+  }
+
   // local constants: general
   const int* Madr = m->dof_Madr;
   const int* parentid = m->dof_parentid;
-  const mjtNum* qLD = d->qLD;
-  const mjtNum* qLDiagInv = d->qLDiagInv;
   const int* simplenum = m->dof_simplenum;
 
   // local constants: island specific