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
2. Add Error checks for the data type (`np.uint8`) and dimension check for image (2 - grayscale, 3 - RGB, 4 - RGBA) and mask (`bool`, `True` for the region to be inpainted)