Start to handle Cartesian domain, mesh and field
Add CartesianVelocityField class

CartesianVelocityField and RLLVelocityField are derived from
StructuredVelocityField.
Lots of changes

Domain class has been templatized by taking CoordType argument.

Use 'auto' when possible, it is convenient.

Add StructuredGridStyle enum (FULL_LEAD and HALF_LEAD) to judge the usage of index.