STL Export on Bulbcalculator

BulbCalculator (in HEAD) has now the export to STL file at least as Ascii. This follow a request from a user to be able to print a model with a 3D printer.
Using this request as starting point I added some features that are likely to be used to mill the model, so the STL can also be exported as half model.

There is, for now, a limitation in this since only one half of the model is saved to the STL file.
If you have a model perfectly symmetrical you just have to mill two times the object and you have a complete bulb. Conversely, if you have an asymmetric model (I presume on the vertical plane) this leads to a small amount of work on the CAD/CAM side since you have to generate a mirrored object.

This limitation will be eliminated in a next version, but for now you should keep it in mind if you plan to print (or mill) a bulb designed with BulbCalculator.