Add Background Fields for Particle Pusher
Add option to run over GUARD too

- particle pusher: we do not want do communicate,
  add the field for interpolations in my own guard
- cellwiseOperation: calculate the right total/physical offset
  transparently for each AREA the user specifies
- Fix Influence Flag of FieldE
Clean up Documention and Interfaces

- use functor
- add/update documentation
- change example to disabled
- use simDim
Add More Documentation and Conversions
Rename and Split Param Files for Fields

- rename: fieldConfig -> fieldSolver
- new file: fieldBackground.param
Rename background field class

Avoids missinterpretation as a field.
Merge pull request #131 from ax3l/topic-bgFields

Add Background fields