Diff pos max modification

The DIFF_POS_MAX metric has been modified to use 3 new thresholds values.

The new definition of DIFF_POS_MAX: 

The difference between the maximal peaks positions. The difference is always a positive value. NA is returned  if minimal peak value is not respected. A profile can have more than one position with the maximum value. In that case, the median positions within a certain range of the maximum value. A threshold  argument can also be set to ensure that the distance between two maximum values is not too wide. When this distance is not respected, it is assumed that more than one peak is present in the profile  and NA is returned.