Convert size() comparisons to using empty

It's easier to read, but in the case of std::list this provides a constant time comparison.

src/asp/Core/BlobIndexThreaded.cc

@@ -161,7 +161,7 @@ void BlobCompressed::add_row( Vector2i const& start,
 void BlobCompressed::absorb( BlobCompressed const& victim ) {
 
   // First check to see if I'm empty
-  if ( m_row_start.size() == 0 ) {
+  if ( m_row_start.empty() ) {
     m_min = victim.min();
     for ( int i = 0; i < victim.num_rows(); i++ ) {
       m_row_start.push_back( victim.start(i) );

src/asp/IsisIO/RPNEquation.cc

@@ -83,7 +83,7 @@ double RPNEquation::evaluate( std::vector<std::string>& commands,
                               std::vector<double>& consts,
                               double const& t ) {
   // Evaluates an equation in the internal format
-  if ( commands.size() == 0 )
+  if ( commands.empty() )
     return 0;
   int consts_index = 0;
   std::stack<double> rpn_stack;

src/asp/Tools/stereo.cc

@@ -184,7 +184,7 @@ int main(int argc, char* argv[]) {
 
   // If the user hasn't specified a stereo session type, we take a
   // guess here based on the file suffixes.
-  if (stereo_session_string.size() == 0) {
+  if (stereo_session_string.empty()) {
     if ( (boost::iends_with(cam_file1, ".cahvor") && boost::iends_with(cam_file2, ".cahvor")) ||
          (boost::iends_with(cam_file1, ".cahv") && boost::iends_with(cam_file2, ".cahv")) ||
          (boost::iends_with(cam_file1, ".pin") && boost::iends_with(cam_file2, ".pin")) ||
@@ -214,7 +214,7 @@ int main(int argc, char* argv[]) {
   bool check_for_camera_models = true;
   if ( stereo_session_string == "isis" ) {
     // Fix the ordering of the arguments if the user only supplies 3
-    if (out_prefix.size() == 0)
+    if (out_prefix.empty())
       out_prefix = cam_file1;
     check_for_camera_models = false;
   }