Improved surface smooth

This work is an improved version of the work done by Marc Lalancette, 2014-02-04.

New features 2018.10.4
1. The new input variable "ViewPoint (default [0,0,0])" will help to control
the normal direction and reduce the error caused by inconsistent
normal direction. If you want to make all normals inward, you could choose a point
inside the closed 3D shape.
2. Through making every edge distinct in one direction to solve the
sparse error problem.
3. Solved an error which the code will not work and return the output with all NaN
in a case when some vertices are not in faces.

With these above improvements, the code is now more general to use.

Improved by Chong Wu, City University of Hong Kong, Hong Kong SAR, China, 2018.10.4

Improved the normal calculation, revised the sparse error, and NaN error. The original work by Marc Lalancette, Toronto, Canada, 2014-02-04 is: https://ww2.mathworks.cn/matlabcentral/fileexchange/45416-surfacesmooth?s_tid=srchtitle