Refactor XYZ base class methods dependent on derived classes

[mferrera]

Some methods in XYZ have a dependency on instances of classes derived from it. For example,

xtgeo/src/xtgeo/xyz/_xyz_oper.py

Line 120 in 6addf60

def operation_polygons_v2(self, poly, value, opname="add", inside=True, where=True):

This function is "part of" the XYZ class but expects poly to be a Polygons instance, which is a class derived from XYZ.

Not clear if just making these abstract methods in XYZ is the best choice, there might be a better one.

There might be other cases of this in which _xyz_something.py modules are imported into subclass modules.