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.  
added bool variable, but the code above is not updated since it does not effect line 58
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