Fix slice field printer plugin

This solves all issues mentioned in #348 

 - command line flags use a `.` now
 - slices are checked if they are in the range of 0.0 to 1.0 and wrong slices are ignored
 - command line argument and variable name for the file name is corrected
 - template specialization is used now to determine the SI unit 
 - `FieldJ` is included

Only the questions, whether this plugin remains in picongpu or hdf5 output only is prefered, has to be answered.  
use `Field::getUnit()[0]` here