Use float for elapsed_time

When accumulating elapsed_time from many moves that take less than 1
second, elapsed_time does not get incremented because (unsigned int)0.9
= 0.

xs/src/libslic3r/GCode.cpp

@@ -198,7 +198,7 @@ Wipe::wipe(GCode &gcodegen, bool toolchange)
 
 GCode::GCode()
     : placeholder_parser(NULL), enable_loop_clipping(true), enable_cooling_markers(false), layer_count(0),
-        layer_index(-1), layer(NULL), first_layer(false), elapsed_time(0), volumetric_speed(0),
+        layer_index(-1), layer(NULL), first_layer(false), elapsed_time(0.0), volumetric_speed(0),
         _last_pos_defined(false)
 {
 }

xs/src/libslic3r/GCode.hpp

@@ -82,7 +82,7 @@ class GCode {
     const Layer* layer;
     std::map<const PrintObject*,Point> _seam_position;
     bool first_layer; // this flag triggers first layer speeds
-    unsigned int elapsed_time; // seconds
+    float elapsed_time; // seconds
     double volumetric_speed;
     
     GCode();

xs/xsp/GCode.xsp

@@ -129,9 +129,9 @@
     void set_first_layer(bool value)
         %code{% THIS->first_layer = value; %};
     
-    unsigned int elapsed_time()
+    float elapsed_time()
         %code{% RETVAL = THIS->elapsed_time; %};
-    void set_elapsed_time(unsigned int value)
+    void set_elapsed_time(float value)
         %code{% THIS->elapsed_time = value; %};
     
     bool last_pos_defined();