ENH: Generate stub-files to support type hints

Wrapped Python classes and methods, have introspection
enabled typehints to allow auto-completion in IDE's.

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

A large number of available ITK classes are currently supported by this
feature, with more being worked on.

The itk-stubs directory is created with a __init__.pyi file
that provides all the interface stubs to tools that
can utilize stub files. The included interface files provide
information needed for IDE's like Pycharm. Once built there should be no
extra setup needed to get the hinting functionality.

The hints for type selection factories use Template and Proxy names
appended to them to differentiate them from the actual ITK classes. The
interfaces may provide hints for types that are not valid in some cases.
The interfaces suggest 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.

NOTE: Stub files must use relative imports for .pyi files in order to
expose the imported symbols.

Co-authored-by: Kian Weimer <kianweimer@gmail.com>

Wrapping/Generators/SwigInterface/CMakeLists.txt

@@ -315,6 +315,9 @@ macro(itk_end_wrap_module_swig_interface)
   set(mdx_file "${WRAPPER_MASTER_INDEX_OUTPUT_DIR}/${WRAPPER_LIBRARY_NAME}.mdx")
   set(module_interface_file "${WRAPPER_MASTER_INDEX_OUTPUT_DIR}/${WRAPPER_LIBRARY_NAME}.i")
   set(module_interface_ext_file "${WRAPPER_MASTER_INDEX_OUTPUT_DIR}/${WRAPPER_LIBRARY_NAME}_ext.i")
+  set(ITK_STUB_DIR "${ITK_DIR}/Wrapping/Generators/Python/itk-stubs")
+
+  set(ITK_STUB_PYI_FILE "${ITK_STUB_DIR}/${WRAPPER_LIBRARY_NAME}.pyi")
 
   foreach(module ${SWIG_INTERFACE_MODULES})
     # create the swig interface
@@ -361,8 +364,9 @@ macro(itk_end_wrap_module_swig_interface)
       else()
         unset(snake_case_config_file)
       endif()
+
     add_custom_command(
-      OUTPUT ${i_files} ${typedef_files} ${idx_files} ${snake_case_config_file}
+      OUTPUT ${i_files} ${typedef_files} ${idx_files} ${snake_case_config_file} ${ITK_STUB_PYI_FILE}
       COMMAND ${Python3_EXECUTABLE} ${IGENERATOR}
         ${mdx_opts}
         ${swig_libs}
@@ -374,6 +378,7 @@ macro(itk_end_wrap_module_swig_interface)
         --interface-output-dir "${WRAPPER_MASTER_INDEX_OUTPUT_DIR}"
         --library-output-dir "${WRAPPER_LIBRARY_OUTPUT_DIR}"
         --submodule-order "${WRAPPER_SUBMODULE_ORDER}"
+        --pyi_dir "${ITK_STUB_DIR}"
         DEPENDS ${ITK_WRAP_DOC_DOCSTRING_FILES} ${xml_files} ${IGENERATOR} ${typedef_in_files}
         WORKING_DIRECTORY / # Arguments to WORKING_DIRECTORY may use generator expressions
       VERBATIM