Add Attractor:
An attractor is defined with the following pattern:
ATTRACTOR(xa;ya;za;a;b) where:
xa;ya;za : coordinates of attractor point
a : desired size on attractor point
b : distance of influence of attractor
Formula computed is following: R-(R-a)*exp(-(r/b)^2)
with r : distance from attractor
------------
Change behavior when checking size map functions.
* If size map is not a valid python function:
- it will not be taken into account at computation,
- the text in the table is colored in white on red background.
* Empty functions are not allowed
