add 32 export in raw

just to export 32 bits images
maybe it's dangerous here... We using genericwriter on value type T, the behavior would be:
* export as raw8 with cast function from T -> char 
* except if T==uint32_t which would be exported as raw32

This is maybe a bit complex/strange for the user... I would say: in the generic wirter/reader, we only consider raw8 export. If the user wants raw32, he/she has to explicitly use the Raw32 method of the raw writer.. what do you think ?