#2 Add support for doublebars (norm)

[Renaud Métrich]

It is currently not possible to write a norm '|| a ||'.
Workarounds exist, such as write two absolute values or typing double pipes characters, but these workarounds are not satisfactory.

Unfortunately, it seems not possible to write a norm using regular characters and a EQ field. The sole solution seems to insert a self-made inline drawing representing a double bar and scaling it.
The current code does not enable to rescale vertically only a drawing.