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.  
The general transformation is a data element of `ValueType` multiplied point-wise with a value of type `UnitValueType`.

It just makes it easier to assign the unit in the same dimensionality as the data element, this is why we use a unit from a vector (componentwise) for a vector field.