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.  
I am a bit confused about `Field::getUnit()`:
This method returs vectors `UnitValueType`. So there is a unit conversion for each dimension, right? (Even if we currently use the same unit system for all dimensions.)

Therefore, `float_X(math::abs(FieldJ::getUnit()));` as currently used, is wrong since it is `sqrt(3)*UNIT`.

If the conversion is by default depending on the diemsion, how should I write a general transformation?