Inpaint NaN using Griddata

Inpaint: Substitute a matrix that has some nan values with interpolated values.

This algorithm is inspired by the work of John D'Errico
https://www.mathworks.com/matlabcentral/fileexchange/4551-inpaint_nans

However, I do notice that the inpaint_nans algorithm given by John D'Ericco gives a higher gradient compared to griddata. So, this algorithm provides an alternative solution that some people may find it helpful.