Add of R functions paths for simulators and observers

* Added functions
* Added tests

(references OpenFLUID/openfluid#183)
Add of get/set/remove functions for parameters

* Added functions
* Added tests
* Sorted functions into R file

(references OpenFLUID/openfluid#183)
Add of removeAttribute function

* Added removeAttribute function
* Added tests

(references OpenFLUID/openfluid#183)
Rename of get/set simulation time step functions

(references OpenFLUID/openfluid#183)