Update mixtureParticleFilter.cpp

I am getting compilation errors when building with clang on Mac. Here the log
```
In file included from /Users/me/ros/catkin_install/src/bfl/src/filter/mixtureParticleFilter.h:375:
/Users/me/ros/catkin_install/src/bfl/src/filter/mixtureParticleFilter.cpp:105:27: error: member reference base type 'Mixture<SVar> *' is not a structure or union
  for(int i =0 ; i < prior.NumComponentsGet() ; i++)
                     ~~~~~^~~~~~~~~~~~~~~~~
/Users/me/ros/catkin_install/src/bfl/src/filter/mixtureParticleFilter.cpp:111:27: error: member reference base type 'Mixture<SVar> *' is not a structure or union
  for(int i =0 ; i < prior.NumComponentsGet() ; i++)
                     ~~~~~^~~~~~~~~~~~~~~~~
/Users/me/ros/catkin_install/src/bfl/src/filter/mixtureParticleFilter.cpp:113:32: error: member reference base type 'Mixture<SVar> *' is not a structure or union
    _old_samplesVec[i] = (prior.ComponentGet(i)->ListOfSamplesGet());
                          ~~~~~^~~~~~~~~~~~~
/Users/me/ros/catkin_install/src/bfl/src/filter/mixtureParticleFilter.cpp:362:16: warning: self-comparison always evaluates to true [-Wtautological-compare]
        result == result && this->DynamicResampleStepOne(i);
               ^
/Users/me/ros/catkin_install/src/bfl/src/filter/mixtureParticleFilter.cpp:451:16: warning: self-comparison always evaluates to true [-Wtautological-compare]
        result == result && this->ResampleOne(i);
               ^
```

To solve this issue I had to change `.` operator to `->` at mixtureParticleFilter.cpp:105, mixtureParticleFilter.cpp:111, mixtureParticleFilter.cpp:113.