Exporting fluids from 4.2 in JSON format is mostly working - done with alphar and alpha0;  All to_json functions take json instances

Had to modify rapidjson double writing since it was not using high enough precision.  Python reads the JSON file very nicely.

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/ibell/coolprop