Add bilinear interpolation in regrid classes

The mesh index of structured mesh is minused by one when boundary
condition is periodic to hide virtual boundary grids from user.

RLLVectorField inherits StructuredVectorField.
Add transformToPS to SphereVelocity class

To clearify the data range, when the boundary condition is periodic, the
virtual boundary grids is outside (0, nx-1) as -1 or nx.
Refactor documentation in Field class hierarchy