Add new mju_threadpool API function, and delete old threading API.

PiperOrigin-RevId: 922838541
Change-Id: Id9f7e0fb298ffde61fcc49a802dc78971858ce51

simulate/simulate.h

@@ -153,6 +153,7 @@ class Simulate {
     std::optional<std::string> save_mjb;
     std::optional<std::string> print_model;
     std::optional<std::string> print_data;
+    bool update_threadpool;
     bool reset;
     bool align;
     bool copy_key;
@@ -205,6 +206,10 @@ class Simulate {
   // simulation
   int run = 1;
 
+
+  // number of workers in threadpool
+  int nthread = 0;
+
   // atomics for cross-thread messages
   std::atomic_int exitrequest = 0;
   std::atomic_int droploadrequest = 0;
@@ -320,9 +325,10 @@ class Simulate {
 
 
   // simulation section of UI
-  const mjuiDef def_simulation[14] = {
+  const mjuiDef def_simulation[15] = {
     {mjITEM_SECTION,   "Simulation",    mjPRESERVE, nullptr,     "AS"},
     {mjITEM_RADIO,     "",              5, &this->run,           "Pause\nRun"},
+    {mjITEM_EDITINT,   "Num threads",   5, &this->nthread,       "1"},
     {mjITEM_BUTTON,    "Reset",         2, nullptr,              " #259"},
     {mjITEM_BUTTON,    "Reload",        5, nullptr,              "CL"},
     {mjITEM_BUTTON,    "Align",         2, nullptr,              "CA"},