Adding documentation

[protaisM]

Here is a list of some parts of the code which lack documentation :

• splines in general (builder, evaluator, interpolator...) -> to what correspond the template parameters ? Maybe an example of how and when to use them ?
• PDI -> how to write a pdi_out file ? What to modify in order to get data out of gysela ? Best practices: should we create a new event or append to a previous one ?
• CPU/GPU -> for each/parallel_for_each ? default execution space in template parameters ?