Improve const correctness.

See https://usfirst.collab.net/sf/tracker/do/viewArtifact/projects.wpilib/tracker.4_defects/artf4148

Change-Id: I47b0d5a91fd49e47e2c7348b0705e998ec815682

wpilibc/wpilibC++Sim/src/Gyro.cpp

@@ -73,7 +73,7 @@ Gyro::~Gyro()
  * @return the current heading of the robot in degrees. This heading is based on integration
  * of the returned rate from the gyro.
  */
-float Gyro::GetAngle( void )
+float Gyro::GetAngle() const
 {
     return impl->GetAngle();
 }
@@ -86,7 +86,7 @@ float Gyro::GetAngle( void )
  *
  * @return the current rate in degrees per second
  */
-double Gyro::GetRate( void )
+double Gyro::GetRate() const
 {
     return impl->GetVelocity();
 }
@@ -103,7 +103,7 @@ void Gyro::SetPIDSourceParameter(PIDSourceParameter pidSource)
  *
  * @return The angle in degrees.
  */
-double Gyro::PIDGet()
+double Gyro::PIDGet() const
 {
 	switch(m_pidSource){
 	case kRate:
@@ -129,7 +129,7 @@ void Gyro::StopLiveWindowMode() {
 
 }
 
-std::string Gyro::GetSmartDashboardType() {
+std::string Gyro::GetSmartDashboardType() const {
 	return "Gyro";
 }
 
@@ -138,6 +138,6 @@ void Gyro::InitTable(ITable *subTable) {
 	UpdateTable();
 }
 
-ITable * Gyro::GetTable() {
+ITable * Gyro::GetTable() const {
 	return m_table;
 }