Simplify state saving in slave_simulator

By simply not allowing state saving when modifiers are active (which is
sketchy anyway), the implementation becomes much simpler.

include/cosim/algorithm/simulator.hpp

@@ -146,8 +146,6 @@ class simulator : public manipulable
      *  `state_index`. The index is only valid for this particular simulator.
      *
      *  The function may be called at any point after `setup()` has been called.
-     *
-     *  \pre `this->model_description().can_save_state`
      */
     virtual state_index save_state() = 0;
 
@@ -157,8 +155,6 @@ class simulator : public manipulable
      *  This function does the same as `save_state()`, except that it
      *  overwrites a state which has previously been stored by that function.
      *  The old index thereafter refers to the newly-saved state.
-     *
-     *  \pre `this->model_description().can_save_state`
      */
     virtual void save_state(state_index stateIndex) = 0;
 

include/cosim/model_description.hpp

@@ -195,9 +195,6 @@ struct model_description
 
     /// Variable descriptions.
     std::vector<variable_description> variables;
-
-    /// Whether saving of state is supported.
-    bool can_save_state = false;
 };
 
 /// Getter for returning a variable description.

src/cosim/fmi/v1/fmu.cpp

@@ -553,31 +553,31 @@ slave::state_index slave_instance::save_state()
 {
     throw error(
         make_error_code(errc::unsupported_feature),
-        "Getting and setting state not supported");
+        "Getting and setting state not supported in FMI 1.0");
 }
 
 
 void slave_instance::save_state(state_index)
 {
     throw error(
         make_error_code(errc::unsupported_feature),
-        "Getting and setting state not supported");
+        "Getting and setting state not supported in FMI 1.0");
 }
 
 
 void slave_instance::restore_state(state_index)
 {
     throw error(
         make_error_code(errc::unsupported_feature),
-        "Getting and setting state not supported");
+        "Getting and setting state not supported in FMI 1.0");
 }
 
 
 void slave_instance::release_state(state_index)
 {
     throw error(
         make_error_code(errc::unsupported_feature),
-        "Getting and setting state not supported");
+        "Getting and setting state not supported in FMI 1.0");
 }
 
 

src/cosim/fmi/v2/fmu.cpp

@@ -78,8 +78,6 @@ fmu::fmu(
                 << vd.name << " will be ignored";
         }
     }
-    modelDescription_.can_save_state =
-        fmi2_import_get_capability(handle_, fmi2_cs_canGetAndSetFMUstate) != 0;
 }
 
 
@@ -634,6 +632,11 @@ fmi2_import_t* slave_instance::fmilib_handle() const
 
 void slave_instance::copy_current_state(saved_state& state)
 {
+    if (!fmi2_import_get_capability(handle_, fmi2_cs_canGetAndSetFMUstate)) {
+        throw error(
+            make_error_code(errc::unsupported_feature),
+            instanceName_ + ": FMU does not support state saving");
+    }
     const auto status = fmi2_import_get_fmu_state(handle_, &state.fmuState);
     if (status != fmi2_status_ok && status != fmi2_status_warning) {
         throw error(