Fix small area flow comp regression and some other small tweaks (#5844)

* Fix regression, always create SmallAreaInfillFlowCompensator model
small_area_infill_flow_compensation is per-region parameter, it's not necessary to iterate though all regions to check whether to create a model.

* Make SmallAreaInfillFlowCompensator robus.
1. handle spaces/tabs/new lines etc
2. don't throw expection, fall back to no-op instead if parsing failed

* Fixing an issue that changing small_area_infill_flow_compensation per modifier didn't take effect

src/libslic3r/GCode/SmallAreaInfillFlowCompensator.hpp

@@ -5,6 +5,7 @@
 #include "../PrintConfig.hpp"
 #include "../ExtrusionEntity.hpp"
 #include "spline/spline.h"
+#include <memory>
 
 namespace Slic3r {
 
@@ -23,7 +24,7 @@ private:
     std::vector<double> flowComps;
 
     // TODO: Cubic Spline
-    tk::spline flowModel;
+    std::unique_ptr<tk::spline> flowModel;
 
     double flow_comp_model(const double line_length);