Optional selective fractions of atoms to represent the "surface layer"

[TonyWu20]

Need to support the selection of which parts of atoms in the model to be considered as the surface. This is to avoid meaningless searching inside the bulk of the model, instead of the "surface" where the real adsorption would take place. However, the actual reachable surface could not be simply designated by only one specific z-height. So the program should accept a specified range of z-coordinates to tell where is the target surface.

P.S. If the selection feature extends to accept range of x, y, z-coordinates, then it is the "local search" inside the model!

[TonyWu20]

Have completed in branch range_search