Updates to AMCL

Updates include 

* Better viewing of the particle cloud - now you can set flag "draw_weight_as_height" and have the particle's z value be set to normalized height. 

*Improved control over init pose covariance - this is right now set from the rviz plugin to a fixed value (too small for the theta). Now you can override this with the flag "use_cov_from_params" and have it use param values std_xx, std_yy and std_tt as the cov values. 

*Also moved std values for motion outside the for loop (they were being recalculated for every particle before sampling)