Create another class of QGField that computes only QGPV and LWA

[csyhuang]

It happens that the stability of $U_{ref}$ can be affected by boundary conditions and resolution. If the user only wants the computation of wave activity, this new class will do the job.

This tasks would possibly involve refactoring/redesigning of existing classes. Think of a solution that would not affect existing users, i.e. not changing interface of any existing class.

[csyhuang]

Given #118 is done, this is no longer needed so I'll close this ticket.