* overhaul pistache templates
* fix function signature in model-source
return type now aligns with definition in model-header
* use default keyword for destructors
* generate pistache samples
* move bin/configs/other/cpp-pistache-server-cpp-pistache.yaml to bin/configs/cpp-pistache-server-cpp-pistache.yaml
* Only generate validation body if necessary
* generate pistache samples
* Fixed#2643
Refactored to/from json functions to use universal object serialization method.
* Code review found incorrect indenting and I forgot to remove unused mustache files.
* Removed helpers class because it is not needed anymore.
* Removed helpers package from docs.
* Reverted helper class removal.
