Refined the `GASMemberOrder` model.
Refined the `GASMemberOrder` model.

* changed some attribute names for readability
* `gassupplierorder` should be a property (`order`), not a foreign key
field attribute
* added `actual_price` property 
* added some comments
kobe25/gasistafelice,michelesr/gasistafelice,kobe25/gasistafelice,matteo88/gasistafelice,michelesr/gasistafelice,feroda/gasistafelice,kobe25/gasistafelice,domthu/gasistafelice,OrlyMar/gasistafelice,OrlyMar/gasistafelice,matteo88/gasistafelice,feroda/gasistafelice,befair/gasistafelice,michelesr/gasistafelice,befair/gasistafelice,kobe25/gasistafelice,OrlyMar/gasistafelice,feroda/gasistafelice,matteo88/gasistafelice,michelesr/gasistafelice,befair/gasistafelice,feroda/gasistafelice,matteo88/gasistafelice,befair/gasistafelice,OrlyMar/gasistafelice