WIP: ENH: Added type hints for wrapped Python classes and methods.

Logic was added to the igenerator.py file to produce type hints for wrapped ITK classes in Python.

Over half of the available ITK classes are currently supported by this feature, with more being worked on.

Two files are created, init.pyi and methods.pyi which contain the hint information needed for an IDE like Pycharm. Once built there should be no extra setup needed to get the hinting functionality.

The hints use have Template and Proxy names appended to them to differentiate them from the actual ITK classes. The hints are not perfect, they are merely suggestions for potentially correct input options. For example methods may only accept a double or character depending on the object type, but the hints will suggest both leaving it up to the user to determine which type is valid.

Wrapping/Generators/SwigInterface/CMakeLists.txt

@@ -270,6 +270,25 @@ set(ITK_WRAP_SWIGINTERFACE_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}" CACHE INTERN
 set(WRAPPER_MASTER_INDEX_OUTPUT_DIR "${ITK_DIR}/Wrapping/Typedefs" CACHE INTERNAL "typedefs dir")
 set(IGENERATOR  "${CMAKE_CURRENT_SOURCE_DIR}/igenerator.py" CACHE INTERNAL "igenerator.py path" FORCE)
 
+# Create/Clear .pyi file
+file(WRITE "${ITK_DIR}/Wrapping/Generators/Python/itk-stubs/__init__.pyi"
+"# Python Header Interface File
+# imports as they appear in __init__.py
+from itkConfig import ITK_GLOBAL_VERSION_STRING as __version__
+from support.extras import *
+from support.init_helpers import *
+from support.types import itkCType, F, D, UC, US, UI, UL, SL, LD, ULL, SC, SS, SI, SLL, B
+\n\
+# additional imports
+from support.template_class import itkTemplate as _itkTemplate
+from typing import Union, Any\n
+
+import itk.methods as methods\n\n")
+
+# Create/Clear .pyi file
+file(WRITE "${ITK_DIR}/Wrapping/Generators/Python/itk-stubs/methods.pyi"
+"# Python Method Interface File\n\n")
+
 macro(itk_wrap_module_swig_interface library_name)
   # store the content of the mdx file
   set(SWIG_INTERFACE_MDX_CONTENT )
@@ -374,6 +393,7 @@ macro(itk_end_wrap_module_swig_interface)
         --interface-output-dir "${WRAPPER_MASTER_INDEX_OUTPUT_DIR}"
         --library-output-dir "${WRAPPER_LIBRARY_OUTPUT_DIR}"
         --submodule-order "${WRAPPER_SUBMODULE_ORDER}"
+        --pyi_dir "${ITK_DIR}/Wrapping/Generators/Python/itk-stubs"
         DEPENDS ${ITK_WRAP_DOC_DOCSTRING_FILES} ${xml_files} ${IGENERATOR} ${typedef_in_files}
         WORKING_DIRECTORY / # Arguments to WORKING_DIRECTORY may use generator expressions
       VERBATIM