Update defaults in OpenHPL.Data

[boerrebj]

To simplify model setup for the most common situations it suggested to do the following changes to OpenHPL.Data:
SI.Pressure p_a = 0.0 "Default calculation is gauge pressure (set p_a to atmospheric pressure if absolute pressure is required)"
SI.Height p_eps = 0.0 "Pipe roughness height (default is smooth pipe)"

[dietmarw]

One additional more sane default would probably be Vdot_0=0