Include Gauge pressure UOMs in PI AF Pressure UOM class
Add gauge pressure UOMs to AF per common practice in industry. As of AF 2017 R2 the pressure UOM class has only absolute pressure members (relative to absolute zero pressure). Most pressure transmitters in industrial applications are calibrated in gauge pressure, relative to local baromteric pressure. This makes for work on every launch. For example PSIG and InWC are too common examples that need the gauge offset. These are small improvements but every AF customer will benefit. UOMs are important and worthy of any incremental fix.