modify field classes with meaningful hierarchy

[Gioacchino Wang]

Old reg field structure:

X_reg field ---- model 1 ---- modle 2

New reg field structure:

X_reg field ---- Galactic Center ---- model 1 ---- model 2 ---- Thick disk ----- model 1 ----- model 2 ----- Thin disk ----- model 1 ----- Spiral arms ------ model 2

total_field = SUM(field_i) model selection should be done once for all at the beginning of the main routine.

In terms of random field, use an opposite design, 1st level derived classes in generator type, 2nd level derived classes in component type. since we usually fix generator type but change scaling factor or ordering factor etc. in different positions, not the opposite.

[Gioacchino Wang]

1. we need to update with a new class named as GalacticStructure, in which describes geometric information of G.C./Thick disk/Thin disk/Spiral Arms, multiple parameterization methods for each structure is allowed (as derived classes)
2. with help of GalacticStructure class, we can modify old X_regfield classes accordingly