Flow solver improvements: now 2nd-order accurate for velocity and pressure

- 2nd order in velocity and pressure (space and time)
  -- Now using a projection scheme that includes pressure in U*
  -- 2nd-order pressure update correction included
  -- Reformulated Physalis method to be consistent with new projection method
  -- Re-incorporated pressure extrapolation for Lamb coefficients
- 2nd order interpolation to grid cell centers for velocity CGNS file output
- Dirichlet conditions on domain boundaries reformulated to be second-order
  accurate
- Incorporated Taylor-Green vortex initialization for flow solver testing
  (commented out)
- Implemented implicit handling of diffusive term for PERIODIC boundary
  conditions only (CURRENTLY TURNED OFF)
- Housekeeping
  -- Incorporated u_star_2_init code into u_star_2 and removed unnecessary code