Bug fix for Rt and Mz min thresholds

Incorporates expansion and min width thresholds for peak Mz and Rt as suggested by J Stanstrup.  Fixes a bug that occurred when only one peak's Mz or Rt needed to be expanded due to failing the minimum width thresholds.  This bug was due to the fact that a 1 dimensional array is passed to rowMeans() when only one peak needs expansion.  This causes rowMeans() to throw an error and terminate execution because it requires its input to have at least two dimensions.