Contact angle observable reimplemented

* implementation of instantaneous contact angles and other geometrical
  parameters
* 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

pytim/observables/__init__.py

@@ -7,7 +7,7 @@
 from MDAnalysis.core.groups import Atom, AtomGroup, Residue, ResidueGroup
 
 from .observable import Observable
-from .contactangle import ContactAngleGitim, ContactAngleGibbs
+from .contactangle import ContactAngle
 from .basic_observables import Position, RelativePosition, Velocity, Force
 from .basic_observables import Number, Mass, Charge, NumberOfResidues
 from .basic_observables import Distance