Add a pkg_pretent section to force the user to set certain variables in make.conf

Specifically, ITK_COMPUTER_MEMORY_SIZE must always be set and ITK_WRAP_DIMS must be set
when python bindings are enabled.
Further cleaup the ebuild:

  - correct indentions
  - remove patented use flag since it is an unused  leftover from the
    itk-3.X times
  - remove some out-commented lines
correct more indentions