Fast Marching Method for Image Inpainting

Tested for gray scale images. Cython implementation in `_inpaint.pyx` and `inpaint` wrapper function and `initialise` function implemented in python in `inpaint.py`.

`demo_inpaint.py` provides an interactive way for image inpainting. Unit tests are provided in `test_inpaint.py`.

TODO:

1. Add support for 3 or 4 channel image - Wait
2. Add Error checks for the dimensions - Done