initial draft of new polyCub-method specific to isotropic functions
improved code structure and efficiency in polyCub.iso
no need to register "owin" as S4-class since we depend on sp which does it
-> avoids the following warning upon package installation:
Warning in .simpleDuplicateClass(def, prev) :
  the specification for class "owin" in package 'polyCub' seems equivalent to
  one from package 'sp' and is not turning on duplicate class definitions for
  this class
- generalized isotropic cubature for isotropic functions with any center
of isotropy (not necessarily at (0,0))
- implemented tests for polyCub.iso
tests of polyCub.iso with analytical intrfr
included "iso" as method in polyCub()-wrapper
improved documentation and changed to version 0.3-0