Added mjData.eq_active user input variable, for enabling/disabling the state of equality constraints.

Renamed `mjModel.eq_active` to `mjModel.eq_active0`, which now has the semantic of "initial value of `mjData.eq_active`".

Fixes #876.

PiperOrigin-RevId: 570410643
Change-Id: Id03171e751377c7cc453f143abee64239ee2e2ed

test/engine/engine_support_test.cc

@@ -369,6 +369,7 @@ TEST_F(SupportTest, GetSetStateStepEqual) {
   // set controls and applied joint forces to random values
   for (int i=0; i < model->nu; i++) data->ctrl[i] = dist(rng);
   for (int i=0; i < model->nv; i++) data->qfrc_applied[i] = dist(rng);
+  for (int i=0; i < model->neq; i++) data->eq_active[i] = dist(rng) > 0;
 
   // take one step
   mj_step(model, data);