Use VIRTUAL_PYTHON_EXECUTABLE for virtualenv python

Consistently use a cmake cached variable to the path for the python
executable in the virtual environment.

Change-Id: Ibf9702f020a8bd4b8133d6841f7168d852fae14b
Remote ORIGINAL_PYTHON_EXECUTABLE cmake variable

Change-Id: Iaed06cb502328790f2eef5ceac9b443d862c0e9f
Creation of Python virtual environment is now a target.

The dist.Python target now correctly depends on the creation if the
virtual environment.

Change-Id: I7e02e9bd78f9200083a86d7f404e43db7addfe07
remove creation of __init__.py file

Change-Id: I493080b3a88f771203425f776122caf90b1f4fba
fix message ERROR argument to SEND error

Additional error reporting improvements when creating python
virtualenv

Change-Id: I486169105adacda85bbfaf16084c1fa58cc81c6d
remove dependency of native python library on setup.py

Change-Id: Ie91daa9f3d204f22a6b5c818605c4e62e758cf58
Converted SimpleITK into a package from a simple module

Due to the way pip installs packages flat, and wheels' different
handling of data_files, the setup.py now installs SimpleITK as a
package which import all for the SimpleITK.SimpleITK sub-package. The
non-normal handling of installation of setup.py is preserved, along
with the namespace usage of SimpleITK.

Change-Id: Id9588061bac5b7d702a62f569886e3b3cc1f2e84
add creation of wheels to the dist target

Also download and install the python wheel package into the virtual
environment.

Change-Id: Ia240fb34a35b2de97b147da27097fd7f3e8de683
Add CMake Option to build Python wheels for dist target

Change-Id: Ibbcd6624a2c1f1f4e5b5ffc336a4bf8d2888deee
The absolute paths of the doc files are not working with setuptools

They are getting munged and being modified causing errors during
packaging.

Change-Id: I0f87fbf094fedea0666801d6eb3b4d38ad622981
Copy native python module to Wrapping directory

On Windows the libraries are produced in different configuration
directories. The setup.py is not supporting remapping these
directories. So the Python Native library is being copied into the
Wrapping directory for testing,installation and packaging as a post
build step.

Change-Id: I8a5915d57dfc313ae0bcf01c405be2830c7db2ac
Merge topic 'SIMPLEITK-372_FixDistTargetWhenTestingDisabled'

960c8627 remove dependency of native python library on setup.py
d0b233a8 fix message ERROR argument to SEND error
9b4f3302 remove creation of __init__.py file
27674919 Creation of Python virtual environment is now a target.
51fffd2f Remote ORIGINAL_PYTHON_EXECUTABLE cmake variable
4d3c096b Use VIRTUAL_PYTHON_EXECUTABLE for virtualenv python
Merge topic 'SIMPLEITK-506_Refactorsetup.pyForWheels'

dfb514d1 Copy native python module to Wrapping directory
4830ed88 The absolute paths of the doc files are not working with setuptools
518b755c Add CMake Option to build Python wheels for dist target
c5dbc0ec add creation of wheels to the dist target
d5b6f6d6 Converted SimpleITK into a package from a simple module