Contact angle observable reimplemented

[Marcello-Sega]

• implementation of instantaneous contact angles and other geometrical parameters
• implementation of ellipsoidal linear and nonlinear fitting, with resolution of contact line location and contact angle function
• new linear least squares and nonlinear fitting of ellipse, with RMSD tested to be lower than the fit to a circumference
• ellipticity constraint imposed for linear and nonlinear fit
• new procedure to remove automatically the liquid/solid interface atoms and obtain the liquid/vapur interface
• more consistent user interface