Operation docs update (#154)

* Operation Interface Enhancement + Clarification
- added set notation to all the geometry operations
- added a warning to say sample_surface=True doesn't work

* minor fix docs

* fix operation_interface.py doc

---------

Co-authored-by: Dario Coscia <dariocoscia@Dario-Coscia.local>
Co-authored-by: Dario Coscia <93731561+dario-coscia@users.noreply.github.com>

pina/geometry/operation_interface.py

@@ -7,21 +7,28 @@ import random
 
 
 class OperationInterface(Location, metaclass=ABCMeta):
+    """PINA Operation Interface"""
+
     def __init__(self, geometries):
         """
         Abstract Operation class.
         Any geometry operation entity must inherit from this class.
 
+        .. warning::
+            The ``sample_surface=True`` option is not implemented yet
+            for Difference, Intersection, and Exclusion. The usage will
+            result in unwanted behaviour.
+
         :param list geometries: A list of geometries from 'pina.geometry' 
             such as 'EllipsoidDomain' or 'CartesianDomain'.
         """
         # check consistency geometries
         check_consistency(geometries, Location)
 
-        # check we are passing always different 
+        # check we are passing always different
         # geometries with the same labels.
         self._check_dimensions(geometries)
-        
+
         # assign geometries
         self._geometries = geometries