Add attributes in Field classes
Use Arakawa grid type to set field

Also fix a bug in TimeLevels
Minor changes
Modify TimeLevels::getNumLevel

When half level is used, we should return the level number without half
levels at default. But user can send ‘INCLUDE_HALF_LEVEL’ (a.k.a. true)
to get the total number.
Fix operator +, -, *, /

In derived class, we should use the following signature to call base
class version.

<derived-class> res;
<base-class> &tmp = res;
tmp = <base-class>::operator+(other);
Add grid conversion in StructuredVectorField

Implement C grid to A grid.