Different vertical output types should be handled within same `Calc` object

[spencerahill]

• Currently, a new Calc is created for each one, and thus the data is loaded and released unnecessarily the second time.
• Both require the exact same data and thus its inefficient to have them as separate Calcs.
• In contrast, 'sigma' and 'pressure' dtype_in_vert should be separate.
• So it boils down to making dtype_out_vert behave like dtype_out_time does rather than the rest.
• This could be addressed during implementation of CalcSuite class, since in that case CalcSuite will load and hold all the necessary data.