Add Background fields

Adds additional background fields.

By this pull request I try to provide a method to the user to specify background fields

![time dependent fields](http://latex.codecogs.com/gif.latex?%5Cvec%7BE%7D_%5Ctext%7BSI%7D%28%5Cvec%7Br%7D%2Ct%29%7E%5Ctext%7Band%7D%7E%5Cvec%7BB%7D_%5Ctext%7BSI%7D%28%5Cvec%7Br%7D%2Ct%29)

in a *convenient* manner. Convenient means, the user can input the position ![r](http://latex.codecogs.com/gif.latex?%5Cvec%7Br%7D) either in cells or in meters. Same for time step and real time in seconds.
He further should be able to think of his/her background fields in their total value in SI units (`V/m` or `T`), too.

I tried to add an example adding a static sinus/cosines with extensive documentation.

**Features:**
- [x] Add Background fields to particle pusher
- [x] extensive testing
- [x] ~~Take care of background fields in field solver (for spatial non-constant fields)~~ -> *later*
- [x] resolve `\todo`'s about the `simDim`, rebase against `dev` after #134 was merged
- [x] document param file more: add option to input values in `_SI` and show conversion to cell numbers and time
- [x] ~~make options where to include more flexible (e.g. "enable for ionization")~~ -> *later*

~~**Do not merge yet, still have to rebase after #135 was merged.**~~