Add initTarget & initFov options
Add default coordinate system: equatorial or galactic.
Transform HEALPix tiles according to default coordinate system.
Add option to choose the coordinate system which is represented by grid.
Add tesselation option(for longitude only).
Optimize rendering by generating geometry/labels only if geoBound have been changed.
Add format option of longitude/latitude label layout("HMS", "DMS", "Deg").