Add optional pressure field output to snapshots (ADIOS2)

[ia267]

This PR adds optional pressure output to snapshots. When enabled, pressure is retained after pressure correction, interpolated from cell-centred grid to vertex grid, rescaled to physical (kinematic) pressure, and written as p alongside velocity field u, v, w. Closes #222

Pressure is excluded from checkpoint files because it's a derived quantity that's recomputed from velocity (it's not required for restarts).

Changes:

• added output_pressure (default .false.) to checkpoint_config_t and namelist parsing
• documented output_pressure in user docs
• updated pressure_correction subroutine to retain pressure when keep_pressure=.true.
• added compute_pressure_vert() to convert pseudo-pressure to kinematic pressure via division by dt
• Added interpl_c2v() in vector_calculus_t (general scalar interpolation from CELL (DIR_Z) to VERT (DIR_X)
• Renamed checkpoint_mgr to io_mgr in base_case
• In snapshot manager build the field list dynamically

Note that when pressure output is enabled, extra memory is used to store pressure (CELL, DIR_Z) and pressure_vert (VERT, DIR_X). If output_pressure is not set (or .false.) then we get the current behaviour.

Tested on TGV case (see output below)